You're assuming votes are public, which is not usually the case. If there's no way to check someone else's vote, there's no way to enforce a voting block.
If the votes aren't public enough to see the ballots, how do you know if a voting district cast the correct number of votes for the number of ballots cast? How do you know if the number of quadratic votes match the sum of sqrts of the individual votes? How do you check to see if anyone's cheating?
If you can see the ballots, you can filter for the ones that match the cartel platform, and count to 100. There may be a voting coordination strategy to find out who cheated. Maybe instead of 100 members, you have 99, and each member votes 2 for a different issue.
If there are enough candidates, you can encode a unique ID in the pattern of your individual votes to prove who you are to somebody reading anonymous ballot papers. I've heard this is possible in Australia with a huge list of candidates and multiple votes.
