I don't understand the technical issue. Regardless of which token is used, you need a depository to interface between in-game and out-of-game (blockchain).

Since all of the M (CK/CKM/currency, whatever you call it) is currently in game, the depository (blockchain) wallet starts 'full' with all of the blockchain tokens (as if the currency started out of game, like XMR or BTC, but were all deposited). As people withdraw, the depository wallet pays them out on-chain.

As always, depository fraud or negligence is going to create problems. Multisig, good transparency/reporting, dividing the wallet between multiple custodians, etc. may all be good ideas.