can you stick two monitors per front wheel or are the receivers above each wheel well? can you somehow rig the rear system to the front then?
edit: looks like it uses the rear window antenna for the receiver. i doubt it actually knows the position of the sensor so as long as they are all in a learned position, i don't see why you couldn't secure a sensor inside of each front wheel. just gotta do it with some damn good glue
that or somehow creating a false atmosphere around the sensor in the rear wheels. something like a baloon or even a small box like you were saying that had the correct pressure. just gotta figure a way of filling that or the tire. drill a new valve?