Maybe instead of usernames, the instances could store/trade… salted hashes of the usernames where the salt is the title or unique identifier of the post/comment being voted on?
I didn’t have time to reply earlier, but I was thinking the same thing, except with the extra step of replacing the username with a unique user identifier randomly generated at signup by the user’s instance and kept secret.
I wonder if there’s a way to prevent people from even knowing that two different votes came from the same user.
I was thinking of a variable-length hash, but if we use a fixed length (which I acknowledge the term “salt” does imply), then I agree.