Very good idea, ive pondered it for some time now, as you say, the rear bumper is just like a big air scoop at the moment, especially once you remove the spare wheel. If built right, it will give you more downforce then any spoiler, and keep you stuck to the road.
My worry was that its going to be so close to the exhaust pretty much the full length of the car. I was thinking of doing it out of carbon and fibreglass, but it would melt in no time. A sheet of Ali that big wouldnt be cheap, and thats why i got no further with it. The other issue, which wouldnt be too hard to get around, was that i didnt want it to be a big water trap, and store water under the car, and encourage it to rust easier.
Im looking forward to what you come up with on it!