Its illegal to make kammback any further than the body. Also its illegal to put any such things on your car which in newer than 1997. So any parts you put there are basicly illegal. In most cases police will understand, but you need to ready to pay some R&D costs made by police.
Good suggestion on the active P&G cruise control. If I would have skills to do it I propably would. You have few choices. Make mechanical system that pushes or pulls the standard gas pedal. That I could build easily. Make electrical verion which would fool the gas pedal positions to ECU. For that I dont have the skills.
But I think I will try to get some engineer to work on it as their graduation study/work
.