Very clever. You could mount the detector in the rear window like many NSXers do, run the cord through the headliner and into the mirror.
Then all you need is to make a "negative stencil" with cutouts for the detector display elements, strip all the factory reflective coating off, stick the negative stencil on it, and recoat everything else with new silver backing.
It sounds pretty time consuming and having the stencil pre-made would be very helpful but it could probably all be done in a day, or a weekend at worst. If anyone makes a stencil, please scan it! Actually the real key would probably to produce them as static decals to ensure a tight seal, perfect alignment and easy removal.