St. Augustine's teaching appears to be: The Grace of Justification (and so supernatural faith and charity) can be obtained by Catechumens before Baptism. Yet, it is only in Baptism that they attain complete regeneration, and this is comparable to being born. And it is only after being born that they attain eternal life. Could it be that those in limbo were awaiting the seal of Christ's Baptism?
Second, the Doctor elsewhere speaks of the Grace of Perseverance. All Catholics know the grace of justification is necessary but not sufficient for salvation. An additional gratuitous grace called final perseverance is needed. God may give this or choose not to. Though He will always give it, to someone who seeks it sincerely, and makes use of the means He has given for it to be attained.
Thanks, Trad 123. From On the Predestination of the Saints, Book II, Chapter 46.
http://www.newadvent.org/fathers/15122.htm "Chapter 46.— A Man Who Does Not Persevere Fails by His Own Fault.But it is said, It is by his own fault that any one deserts the faith, when he yields and consents to the temptation which is the cause of his desertion of the faith. Who denies it? But because of this, perseverance in the faith is not to be said not to be a gift of God. For it is this that a man daily asks for when he says, Lead us not into temptation; Matthew 6:13 and if he is heard, it is this that he receives. And thus as he daily asks for perseverance, he assuredly places the hope of his perseverance not in himself, but in God."
St. Augustine's authority is the main reason I rethought my former position: I believe St. Augustine's teaching is most likely true. Justification often before Baptism, but salvation after receiving the Sacrament, which God will Provide, seems a quite solid teaching.
My earlier position was similar to that of St. Thomas, St. Robert and St. Alphonsus. It seems St. Alphonsus and St. Robert may not have disagreed with St. Augustine, if the precise manner in which the Doctor of Hippo laid out his position had been raised to them.
St. Robert admits it is a difficulty to explain how catechumens are saved. It seems best to say catechumens will be saved after Baptism. Because they desired it, God provided it, and thus they were saved, not only upon being conceived, per the example, but actually born. That seems to be the position of the Monks and Nuns at St. Benedict's Centre as well.
One of St. Augustine's other statements, "Baptism is administered invisibly to one whom not contempt of religion but death precludes" doesn't preclude that Baptism could be administered to that person invisibly in some extraordinary way, it is quite consistent with it.