someone on the irc said it can be done even better using
multisig:
This can all be done in a way that doesn't let the service steal. You go to the service and say "I'm mixermember bob, here is an input of mine I want to anonomize, and here is a blinded token for you to sign". The service collects many such inputs during a round. Then randomly later, you unblind the signed token he gave you in step 1... and you return and say "I'm some anonymous guy with a signed token. Add this output". then once all the inputs and outputs for a round have been collected, the mix drafts single transaction that spends all the provided inputs to all the provided outputs and gives it back to all the parties to sign. If everone is happy with it (they've been paid as requested).. they sign. the mixer would enforce quantized output sizes. the accounts cost money to get— thus my "I'm mixmember bob", and refusing to sign gets you banned.