Nothing is stopping you from asking to pay for the room, now is it? Granted, Marriott does not act like McDonald's in trying to push the room UG (Would you like fries with that?) but if you want to pay for the room, have at it. What you're asking is the room be given to you at the hotel's loss of potential profit. If there are several rooms, the hotel should UG you, but if there is one room, it's understandable the hotel would consider it not available for UG.