It's only going to tumble until you hit 10 or 15 mph, after that it would plant the key on the wall pretty steady. Not sure that would be the best place I could think of for a spare key, but it would be covered by the hubcaps and still be accessible with only moderate tools, so maybe?
Funs tory, a friend in high school always locked her keys in the car. We convinced her to hide a spare, so she did. Under the hood. Still had to call her dad out to unlock it...