There are still some gaps to fill. Robin wants to use this to create a federation which cannot steal your money as long as one of them is honest. Here is a design I think is close to working: the federation members would all act as verifiers who agree to challenge a prover (who holds all sidechain deposits) if he doesn't process withdrawal requests correctly.

As long as one of them does so honestly, that honest party could take the prover's money and distribute it to withdrawer's honestly if the prover does not do so.

If this design worked it would improve the trust assumptions of federated sidechains, which currently rely on an honest majority, so that instead they only rely on a single honest party -- which could be you. You could just be one of the verifiers, and then you only need to trust yourself (and bitcoin's standard trust assumptions, e.g. 51% of miners are not censoring your transactions)