It would be dubious. It is a fine line to walk since gen 1 may or may not be fully in the public domain.
Can I talk about what went on at Los Alamos even though there have been movies about it and information on nuclear physics is in the public domain? I don't think that is possible.
So, to circle back to the point about withdrawing clearances from people no longer in the employ of the govt....it should have relatively little meaning. Brennan really can't talk about much beyond what he had for lunch without violating his oath.
True, but violating oaths is becoming much more common and like mentioned already - not prosecuted unless it is part of some other agenda.