I do like the idea of this. However:
A friend did this a few years ago, think it was about 2004/05 and it was at the time Extra Dry or may be it was heineken had their Bottle Return ads, the car had a big:
return your empties here logo on it. Every few days she'd wake to find a few empties on the roof of the car, then one day she came out and the entire car was covered. I've tried google images to find a image of it as I know she took photos, but sadly can't find it. Will ask if anyone else has it. She was working for the advertising company at the time.