It would have to be on the BC level, not the frontend. I don't think that it could happen on steem, which is why I mentioned a tribe or SMT could test it - perhaps for new signups.

I'd be curious to see what would happen if only down-votes were made to be anonymous.

This would be interesting too. The threshold would also be an interesting way to protect smaller users, but I wonder if it would end up in a stream of bots.