Did you see the one which used an electromagnet to hold fake leaves in place? If they got pulled over, they could push a button which would allow the leaves to fall off.
Leaves are not ferromagnetic, so they won't stick to an electromagnet.
A few small holes with a small pump that constantly sucks the air from them would help stick a real, unmodified leaf to the surface. and release it at will. This would require tampering with the license plate, even though in a very minor way.