There might be possible technical solutions to this using hashing. Hashing is like encryption in that the original cannot be extracted, but the hashed result is unique.
For example, a solution would be to have a VOTES table with an indexed column that is a hash of a combination of the user ID, post ID, (and perhaps another "salt", not sure). When a vote is made, the VOTES table is checked that the record (vote) does not already exist, gets an insert, and then a COUNTER is triggered for the actual vote count. (COUNTER is a db command that simply updates a counter). The hash would prevent multiple votes from the same user (as the salted hash is unique), and it would also prevent identifying who the user is from the table.
I really don't even think the votes table would need to itself be federated; it could just be on the user's instance. Upvote/downvote would be a call, but it should really only require the post or comment ID and voter instance. If an instance spams votes, those upvotes/downvotes could be deleted and the instance defederated