I just came across the following link:
Wohin mit übriggebliebenen Wiesn-Marken?
I don't have any leftovers myself, but it seems most tents give you an option to trade them in or use them at their restaurants.
Link is in German (couldn't find an English equivalent), but you should at least be able to match a tent name to an address. Looks like most places will only honor them until the end of October, though.
