You'd have to know which site each hash relates to, so that you know who to give the money to. So the hashing wouldn't gain anything, since the system only works if they can reverse the hash.

It can work for Safebrowsing because they can just have a gigantic list of hashes and it doesn't matter which specific site the user is visiting, only whether it's any site in the list. But for Flattr to work, you need to know which specific site it is.