Here's an idea:
Use microwallets. Like say use dobits (.001 BTC) with a wallet cap.
When people deposit their bitcoins 99.9% gets distributed throughout the game. Under rocks with challenges attached for example. They get an equal amount of dobits in their in-game wallet. As people find the distributed coins they move out of the microwallet of the user who deposited them. If the user transfers dobits to someone else they move from the place they were scattered. Coins spent at non-player character shops go back into the game.
As a result you are using the game itself as a bank making it impossible to steal any of the coins easily. As long as coins are locked or behind an unsolved challenge then they cannot be moved. If they are transferred between in-game wallets without a matching transaction any attempt to withdraw will have them sent back to the owner.
This adds a curious possibility: You can have campaigns where people can choose to play a thief character and steal the dobits but never actually own them. They can then be exchanged for items at shops but the ownership remains with the original user.
This allows the tactics and strategy of spies and thieves to be played out in a controlled way that's challenging and fun rather than allowing people to steal large amounts.
If someone were to hack into the system they would only have access to the dobits. Not worth the trouble.
If people want to withdraw their bitcoins, they will be removed from the game.
If people do a transaction of bitcoins it must be authorized by both parties, even a third (say a guild secretary).
If someone sends the dobits to their in-game wallet by hacking and tries to withdraw, the coins will actually return to the correct owner. Again not worth the trouble to hack.
In order to steal, the attacker has to move the dobits into their wallet, hack into the user's email account, hack intto the secretary's account, or create a fraudulent transaction in the database after hacking into that. And that would require modifying multiple tables.
I could draw up the code for this scheme and it would make the security of the exchanges pale by comparison. In fact, it might replace them.
