The idea of some sort of salvation being possible after death is related, I think, to C. S. Lewis's idea of time. At the very end of The Great Divorce the narrator mentions something related to Calvinism and predestination. Macdonald rebukes him by saying that this implies God is confined to time (sorry, don't have my copy of The Great Divorce here--will find the quote later). This is very sigificant.
God sees all points of time existing at once. What we define as time really does not exist for God. We think of death as being a demarcation point, but is this necessarily true? It is not so from the Divine perspective.
So I think for Lewis, such a thing as a person coming to faith, or to full faith, after death would be possible because of his idea of time as having no limits for God.
This, also, Theo, is, at least to me, one of the chief errors of Calvinism. Besides making God an "unreasonable tyrant," as one British theologian put it, Calvinism assumes that God is in time--that at a point in time he chose some for salvation, some for hell. But time does not exist for God. He sees every point in time as a present moment so it is impossible for him to make a decision at a given point in time. There are no points in time for God. I will find the quote in the The Great Divorce and post it. This is exactly what Macdonald is getting at in that section of the book.
Protestants sometimes use the term "eternity past" to talk about God's foreknowledge, etc. But "eternity past" is a tautology. With eternity there is no past or present, only the continuum of time that God sees as constant. This notion of time had a lot of influence on Lewis's thinking about the afterlife.