Not really. Mental reservation, as traditionally understood, is making a statement that is not false in and of itself, but requires interpretation to be understood in the sense that the speaker intends it.
You were clearly not in a position to honor the customer's request, so the mental reservation of "we have no eggs" could be understood as "I am not capable of getting eggs for you right now". I found as well in retail that shoppers have the execrable habit of seeing someone who is clearly busy with other things, and pulling the person away from their task, as I always told my workers, "people like what busy looks like". If it is a worker who is just standing around idly (and in my store, there was always enough to keep everybody busy, if nothing else you can clean, bring merchandise to the front of the shelf so as not to have gaps in the display, and so on), people are actually less inclined to approach them --- it is as though they like the idea of interrupting someone who is up to their ears in work. Maybe the slackers on my payroll figured this out and thought that they could get out of even more work by goofing off. I don't know.
You learn a lot about people by working retail.
Your understanding of the use of "mental reservation" seems flawed. Please read the following and tell us which category of "mental reservation" you are recommending to the OP in the matter of "we don't have any eggs" in the grocery store:
https://www.gutenberg.org/cache/epub/35354/pg35354-images.html
2399. Mental Reservation.—Mental reservation is an act of the mind by which a speaker restricts or limits his words to a meaning which they do not naturally or clearly convey; or it is an internal modification of an external speech delivered without any or without clear external modification. There are two kinds of mental reservation.
(a) Strict mental reservation is that in which the internal modification is manifested by nothing external, neither by the natural sense of the words (i.e., the meaning that ordinarily attaches to them) nor by their accidental sense (i.e., the meaning they receive from their context, such as the circuмstances of time, place, usage, person who questions, person who is questioned, etc.). Example: Titus, who struck Balbus with a club, denies that he hit him, meaning that it was the club which hit Balbus directly.
(b) Broad mental reservation is that in which the internal modification can be perceived, at least by a prudent person, either from the natural sense of the words (because they are known to be capable of different meanings), or from the context (because circuмstances indicate that the words are not to be taken in their obvious sense). Example: Claudius accidentally ran against and wounded Sempronius and the latter thinks that someone struck him a blow. Claudius denies that he struck Sempronius, or declares to those who have no right to ask that he knows nothing about the matter.
2400. Lawfulness of Mental Reservation.—(a) Strict mental reservation is unlawful and has been condemned by the Church (see Denzinger, nn. 1174-1178). The reasons are, first, that it is a lie, since it employs words that do not at all express what the speaker has in mind, and his mental reservation cannot give them a significance they do not possess; secondly, that, if it were lawful, every dishonest person could easily escape the guilt of lying and yet deceive at will. According to Scripture the sophistical speaker is hateful (Ecclus., xxxvii. 23), but the just man speaks and swears without guile (Ps. xxxiii. 14, xxiii. 4).
(b) Broad mental reservation is unlawful when there is a reason that forbids its use, or when there is no sufficient reason to justify its use. Reservation is forbidden when a questioner has the right to an answer free from all ambiguity, for example, when a pastor questions parties preparing for marriage, when a person who is about to be inducted into office is asked about his freedom from disqualifications, when a witness in court is interrogated about matters on which he can testify, when one party to a contract seeks from the other necessary knowledge about the contract; for in all these cases injury is done by concealment of the truth. Reservation is not justified, unless it is necessary in order to secure some good or avoid some evil, whether spiritual or temporal, whether for self or for another, and the end compensates by its importance for the deception that may be caused. Apart from such necessity mental reservation is, to say the least, a departure from the virtue of Christian sincerity or simplicity, which pertains to truthfulness and which forbids one to conceal the truth from others when there is no good reason for concealment (Matt, v. 37). Moreover, the friendly relations of mankind would be impaired if it were lawful to speak equivocally even when trifling things are discussed or when there is no reason to be secretive.
(c) Broad mental reservation is lawful when there is a sufficient reason for it, such as the public welfare (e. g., the preservation of state secrets or of military plans), spiritual welfare (e.g., the prevention of blasphemy or intoxication), bodily welfare (e.g., the prevention of death or murder), or financial welfare (e.g., the prevention of robbery). But the reservation must be necessary, as being the only lawful means that will secure the end (e.g., one should not use reservation when evasion or silence will suffice); and it should not be injurious to the rights of another (e.g., it should not be employed against the common good, in favor of a private good). The reason for the present conclusion is found in the principle of double result (see 103 sqq.) and in the fact that a broad mental reservation is not intrinsically evil, since it contains no lie or insincerity and causes no injury to individuals or society. There is no lie, because the words correspond with the thought, either from their natural signification (in case of double-meaning words), or from their accidental signification (in case of words whose meaning is varied by the context); there is no insincerity, for the aim is only to conceal a truth that should not be made known; there is no injury to the listener or questioner, since, if he is deceived, this is due to his own heedlessness or dullness or unjustified curiosity; there is no injury to society, since the general welfare demands that there be some honest means of eluding unjust inquiries and of protecting important secrets. Our Lord Himself, who is infinitely above all suspicion of duplicity or insincerity, may have used broad mental reservations when He declared (John, vii. 8-10) that He would not go up to Jerusalem, that the daughter of Jairus was not dead but sleeping (Matt., ix. 24). Other cases of mental reservation in Scripture are found in Eliseus (IV Kings, vi. 19).
2401. When Is Broad Mental Reservation Lawful?—There is general agreement that broad mental reservation is lawful in the following cases:
(a) it is lawful and obligatory when one is bound to keep the truth from the person who asks it. Hence, those who are questioned about secrets which sacramental or professional confidence forbids them to disclose (e.g., confessors, doctors, lawyers, statesmen, and secretaries) should deny knowledge, or, if hard-pressed, even the facts. The answer, “I do not know” or “No,” in these cases simply means: “I have no personal or communicable knowledge.” In war time a government has the right to censor the news in order to keep information from the enemy. A reason of charity might also make it obligatory to disguise the truth by mental reservation (e.g., when a clear reply given to the question of a sick person would only weaken a slender hope of saving his life, or when exact information given to a gunman would enable him to overtake an intended victim);
(b) it is lawful when a reasonable local custom permits one to withhold the truth. Thus, an accused person, even though guilty, has the right to plead not guilty, which means that he does not confess guilt; a person who has a visitor at an unseasonable hour may send word that he is not at home, which means that she is not at home to visitors, a person who is asked for an alms or a loan which he cannot conveniently grant may answer, according to many, that he has not the money, which means that he has no money to spare for those purposes (see 2251).
2402. Ambiguous Answers.—Are ambiguous answers which are not given according to the questioner’s mind, and for which there are no reasonable justifications, to be classed as lies?
(a) If the answer, even in the setting of its context, retains its ambiguity or can be interpreted in two ways, there is not strictly speaking a lie, for the words signify, though obscurely, what is in the speaker’s mind. But this is a form of insincerity known as equivocation or quibbling, which many regard almost as disreputable as plain lying. The pagan oracles that made predictions that would suit any turn of events and politicians who so word themselves as to be on opposite sides at the same time are examples of equivocation.
(b) If the answer, though verbally susceptible of two senses, is contextually limited to one sense, it is a lie; for it does not express the speaker’s mind. Thus, if Titus knows that Balbus is good physically or mentally but not morally, he equivocates by answering that Balbus is good, if from the circuмstances this indicates only that in some way or other Balbus is good; but Titus lies by answering that Balbus is good and restricting his meaning to physical goodness or industry, if the question propounded referred to moral goodness.