My store actually have the same problem, it seems like this game is so rare now days. It only come in small shipments at a time and many customers didn't receive their copy of this game from previous promos.
Another alternative was, if the customer didn't open the MGS4 game with the promo, exchanging the give-away game to match the current flyer week promo was possible if it's the same bundle and within the 30days of purchase. This method might save you some hassle instead of waiting for a single game and not to lose your promo to just $5 worth of savings when your raincheck expires.
If I remember correctly, some of the old promos has an option of a substitute game if one of the game is sold out. If FS is able to make something happen, it would resolve some of the backlog of rainchecks on GOW3.
