Been thinking about this a bit and I think this would be a great solution for us Elise/Exige owners. I am going to work on a design that would incorporate rare earth magnets that would be glued to the inside of the plate mount area (behind the fiber glass).
Additional magnets would be glued to the back side of a plate and a thin film or felt pad would go over this to keep the magnet from directly touching the fiber glass. I think it may work well and be non intrusive.
If there is additional interest I would share my findings on parts and stuff, I don't think it would be all that hard to figure out.