Not a clue, but it wouldn't be hard to cobble one togeher with bits from the home improvement store. In fact, just about any magnetic "thing" can be placed on the hood when your bikes are up top and will serve the same purpose.
Are you worried about the garage at home or overhangs while away from home? If it's at home, make a sign that says "Got Bikes?" and hang it about a 1' high on the backside of your garage door from two short pieces of chain or wire. When you open the door, the sign will hang down in plain view (pat. pending - I should manufacturer and sell these).
If you're concerned about drive-through joints, lay a white paper sign on your dashboard with the word BIKES UP in black printed in mirror view. It will reflect into your windshield like a heads-up display. Just don't forget to put it away when you don't have bikes otherwise your subconscious will ignore it.