This passage from Aquinas is riddled with mistakes.  Bases it on the authority of St. Augustine, without realizing that Augustine retracted the opinion.  And Augustine cited Cyprian, who reasoned falsely from the Good Thief because Baptism had not been made obligatory yet.

St. Thomas Aquinas, Doctor of the Church (13^th century): Summa Theologica, Whether there are two ways to be distinguished of eating Christ's body?
“Consequently, just as some are baptized with the Baptism of desire, through their desire of baptism, before being baptized in the Baptism of water; so likewise some eat this sacrament spiritually ere they receive it sacramentally.”

Whether a man can be saved without Baptism?
“Secondly, the sacrament of Baptism may be wanting to anyone in reality but not in desire: for instance, when a man wishes to be baptized, but by some ill-chance he is forestalled by death before receiving Baptism. And such a man can obtain salvation without being actually baptized, on account of his desire for Baptism, which desire is the outcome of "faith that worketh by charity," whereby God, Whose power is not tied to visible sacraments, sanctifies man inwardly. Hence Ambrose says of Valentinian, who died while yet a catechumen: "I lost him whom I was to regenerate: but he did not lose the grace he prayed for."

Whether grace and virtues are bestowed on man by Baptism?
Reply to Objection 2. As stated above (1, ad 2; 68, 2) man receives the forgiveness of sins before Baptism in so far as he has Baptism of desire, explicitly or implicitly; and yet when he actually receives Baptism, he receives a fuller remission, as to the remission of the entire punishment. So also before Baptism Cornelius and others like him receive grace and virtues through their faith in Christ and their desire for Baptism, implicit or explicit: but afterwards when baptized, they receive a yet greater fulness of grace and virtues. Hence in Ps. 22:2, "He hath brought me up on the water of refreshment," a gloss says: "He has brought us up by an increase of virtue and good deeds in Baptism."

Whether the Baptism of Blood is the most excellent of these?
"The shedding of blood for Christ's sake, and the inward operation of the Holy Ghost, are called baptisms, in so far as they produce the effect of the Baptism of Water. Now the Baptism of Water derives its efficacy from Christ's Passion and from the Holy Ghost, as already stated. These two causes act in each of these three Baptisms; most excellently, however, in the Baptism of Blood. For Christ's Passion acts in the Baptism of Water by way of a figurative representation; in the Baptism of the Spirit or of Repentance, by way of desire. but in the Baptism of Blood, by way of imitating the (Divine) act."

Whether three kinds of Baptism are fittingly described--viz. Baptism of Water, of Blood, and of the Spirit?
Consequently, a man may, without Baptism of Water, receive the sacramental effect from Christ's Passion, in so far as he is conformed to Christ by suffering for Him. Hence it is written (Apoc. 7:14): "These are they who are come out of great tribulation, and have washed their robes and have made them white in the blood of the Lamb." In like manner a man receives the effect of Baptism by the power of the Holy Ghost, not only without Baptism of Water, but also without Baptism of Blood: forasmuch as his heart is moved by the Holy Ghost to believe in and love God and to repent of his sins: wherefore this is also called Baptism of Repentance. Of this it is written (Is. 4:4): "If the Lord shall wash away the filth of the daughters of Zion, and shall wash away the blood of Jerusalem out of the midst thereof, by the spirit of judgment, and by the spirit of burning." Thus, therefore, each of these other Baptisms is called Baptism, forasmuch as it takes the place of Baptism. Wherefore Augustine says (De Unico Baptismo Parvulorum iv): "The Blessed Cyprian argues with considerable reason from the thief to whom, though not baptized, it was said: 'Today shalt thou be with Me in Paradise' that suffering can take the place of Baptism. Having weighed this in my mind again and again, I perceive that not only can suffering for the name of Christ supply for what was lacking in Baptism, but even faith and conversion of heart, if perchance on account of the stress of the times the celebration of the mystery of Baptism is not practicable."

"...Having weighed this in my mind again and again, I perceive that not only can suffering for the name of Christ supply for what was lacking in Baptism, but even faith and conversion of heart, if perchance on account of the stress of the times the celebration of the mystery of Baptism is not practicable."

It is clear here that St Thomas only considered bod a possibility, showing his uneasiness, having weighed it time and again. Fortunately for him, and us, Trent since clarified that you cannot have Baptism without water, and you cannot have salvation without Baptism.

The New Testament, translated to English at the College of Rheims, 1582 (16^th century): Annotations for John Chapter 3: "Though in this case, God which hath not bound his grace, in respect of his own freedom, to any Sacrament, may and doth accept them as baptized, which either are martyred before they could be baptized, or else depart this life with vow and desire to have that Sacrament, but by some remedilesse necessity could not obtain it."

Maybe so...  But the words of Saint Augustine are certainly wavering.  He had to weigh Cyprian's argument again and again because he couldn't point to any Church teaching on a BOD - He's merely stating his opinion, based on the opinion of another fallible human being.

Also, it should be noted that by considering the example of the good thief, we can already see where St. Cyprian has erred in his argument.  Of course, we know that the Sacrament of Baptism wasn't obligatory until AFTER the resurrection, so the good thief died under the old law.    
  

Sounds reasonable. From a practical POV, since God is the author of the Sacraments, why would He be only able to work within the confines of the sacraments. It seems that in certain circuмstances, he could offer his grace outside of the Sacraments, if He so chooses.