Ok, criminal masterminds, since bitcoin is supposedly only good for criminal deals, what would be the best way to use it while in prison?

I've seen those little paper wallets that are folded over and sealed, so that you can't see the private key. Maybe prisoners could trade those and just write on each one how much is in it. If one prisoner had a contraband android even without net access, he could verify the balances for everyone else as the local broker. And he could synchronize with the main chain via flash drives that are regularly smuggled in, or through a trusted guard...

Is there a better way? What are the risks, besides the obvious?