He should simply be able to explain the mistake and request MtGox to return the funds at least the first time he makes such a mistake.
Isn't the problem here that Gox uses a shared wallet. so the address the SD winnings were sent to could well have been a different Gox customer's deposit address. That customer could already have withdraw Steven's winnings.
Yes, I think there is only a little chance for MtGox to resolve this unless they pay it out of their own pocket. Which would be a cool move for customer satisfaction but will probably never happen (Understandable, as it is really not a MtGox mistake).