An interesting question. Nevertheless, unless MtGox codes become generally accepted as a payment method, there's no need to worry about FRB.
FRB is possible for Gox even without gox-codes being used as currency. They could simply use the bitcoins in deposits, so the sum of users bitcoin balances would be lower then the sum of bitcoins held by gox. That would be a kind of fractional reserve banking, right? The money would not be in circulation, but it would exist as depostis in mtgox accounts with no bitcoin backing.
I doubt they would, because, quite frankly, there is no good and safe way to invest these bitcoins otherwise.
