How about a twist of the original idea that will make it much simpler.

Have "stand-by" award tickets.

If you redeem one of these awards and there is a empty seat in the cabin for which you have a stand-by ticket you get to fly, otherwise you get to keep your stand-by ticket for a use at a later time.

The concept and the risks are known to most of the flying public and this can give both the Airline and the traveler additional flexibility.