I do believe Theymos said permaban appeals would be considered on a case-by-case basis, which implies that not everyone would be treated as equals--because not every case is the same, and not all users contribute equally.  I happen to think that's fair, though I understand that it appears as though some members are getting special treatment--because they are.

The majority of plagiarism offenses are perpetrated by members just trying to pad their post count for sig campaigns, and if they get caught they just move on with a different account after their ban appeal fails to attract attention.  Usually such members have contributed nothing but shitposts and noise to the forum, and there should be no set of criteria which allows them to be unbanned.

And then you get members, usually high-ranked ones, who've made the "mistake" of plagiarizing something early on but who've never repeated that behavior, and somehow their copy/pasting got caught by the plagiarism bot or a very astute member.  If such a member has proven to be a valuable contributor to the forum, I think all evidence of that should be taken into account when Theymos decides to lift the ban or not, and I think that's exactly what he's doing right now.

Those aren't bad guidelines, but there probably wouldn't be much of a practical difference if they were implemented, or it could backfire.  On the first point alone, if a shitposter got banned for plagiarism, he wouldn't get much support from other members or he could employ a team of alt accounts or "friends" to rush to his aid. 

I think it's probably best to just accept that Theymos is going to be the arbiter of ban-lifting.  It's a centralized solution, but he's pretty level-headed and knows what he's doing.  And he's the admin, too.