Well, it's been over 50 years since I first heard these things, but as I recall, it was covered in what we used to call the "13th" session of PFAL.
After the 12th session, in which students *manifested*, there was a separate session on the unforgivable sin. Supposedly, this involved the seed of Satan being irreversibly instilled into people. Born again of the wrong seed, as it were. It was supposed to be the polar opposite of the new birth, and quite different from possession, which could be exorcised. There was lots of talk about many of the people of great influence being "seed boys", and, therefore, irredeemable.(prominent scientists, actors, musicians, world leaders and so forth) Of course, it freaked out lots of people and negated the euphoria of session 12. That's probably why it was eliminated and not revisited until it resurfaced in the Advanced Class teachings. I think it might be included somewhere in the original PFAL collaterals.
edit: You can't really accept it as being possible without first believing in "once saved, always saved" because of the element of permanence.
It's hard to look back at some of this stuff and not laugh at what a chump I must have been.