I don’t have experience with systems like this, but just as sort of a fusion of a lot of ideas I’ve read in this thread, could some sort of per-instance trust system work?

The more any instance interacts positively (posting, commenting, etc.) with main instance ‘A,’ that particular instance’s reputation score gets bumped up on main instance A. Then, use that score with the ratio of votes from that instance to the total amount of votes in some function in order to determine the value of each vote cast.

This probably isn’t coherent, but I just woke up, and I also have no idea what I’m talking about.