Would it be possible to hold bitcoins and use them to trade in a way like how blockchain.info has their wallet service setup? This would put everyone who deposits money in control of their own money in case the site goes down. If it is hacked, the private keys could be revealed so you would need to do the interaction with the wallet client-side. The downsides to this are that if the site goes down, the money in open positions (which would have to be moved from the clients wallet to a communal one) could be lost, and it would lead to massive bloat on the blockchain. To be effective (if ignoring bloat) the system would have to rely on 0 confirmation transactions.