I don't think that this is a good idea, as there is no way to know who is spending what. For example, two friends are sharing a room and are paying for their own trips - but only one credit card can be used to secure purchases. In this case, they would chose not to charge anything to their room and pay as they go on their own. This doesn't mean they are spending less than anyone else, just not charging to the room.
Unfortunately, those using cash or credit cards not tied to the room can't be tracked - unless they gave you some type of card to carry that could be swiped when you made purchases - but I don't see that happening when KTTW cards are so easy.