Jerry loves a good redemption story. Don't put it past him to be the guy that makes Johnny's redemption possible.
I wouldn't rule it out entirely either. But I would say that the odds would be extremely long.
But I think the Cowboys do want to acquire another young quarterback and, at 24, Manziel still fits that description. And the cost to take a shot would be nothing more than PR nonsense, which Jerry Jones and the Cowboys already have in spades anyway.
But, having said that, I do think that Garrett and Stephen would again talk Jerry out of it, even if he was considering it.