It's definitely an interesting benefit, but for everybody who has booked an award stay without having the points (and who might cancel later on), there might be somebody who really wanted the room and can't find the award space.

But to the OP, just book the room, then sort out the points later. The certificate system is rather weird, but you'll get used to it - just don't forget you have an award stay booked and end up paying the rack rate or best flexible as a no-show penalty!