I think there are at least two things going on here.
First, for really dirty laundry, where anonymity is muchos important, you can do as other have suggested and contact the team on the QT.
Second we have a tradition of judging the content, not the user. And I figure that the long timers--the high rep types--are the most likely to value and live that tradition and they are also the bulk of the regulars on meta. So I think that meta is at least moderately trustworthy when it comes to hanging out the issues even when the parties are identified.
And it is important that the actual cases of possible trouble be examined. Real situations are almost always messier than anonymous generalities.
It is worth thinking about how we could have a impartial, trustworthy entity to judge these things, but I don't see an obvious solution.
So that's not the answer you were looking for, but I would suggest that a pattern of holding past misbehavior against a reformed user is cause for censure at some level. I'm not sure how exactly that should be handled, but we need a mechanism.
Flagging the mods of emailing the team seems to be what you've got for now.