I reckon you should still do an in-game crypto coin, just pick one of the others. Risk of attack can be mitigated by accessing the wallets only in game, shirley?
It's a bit too late now. This project is already under way.
But the problem in theory still remains. It's that we'd have to keep their private keys somewhere, and it's then it's our responsibility to secure them. Either we keep the keys on our servers, but then our servers become a target. Or we keep them on the client, but then those files become the target for trojans and whatnot.