Did I understand correctly that someone is contemplating mounting the spare tire on the roof?
Poor weather, rain, ice, wind, heavy truck traffic, big flapping yellow raincoat, flashlight in one hand, wrench in the other, poor attitude of vehicle (tilted, possibly in ditch) come to mind as one is trying to wrestle the spare to the ground when necessary. Tires bounce and roll too if dropped or tossed, usually human bodies don't. Be careful.
It's been a long time, but I've seen spares mounted on the front bumper. Also tilt down racks that lay the tire almost on the ground for you (but that was on the back bumper, darned door).
Could one fabricate a tire rack on the rear step that folds downward with the step? Or something like that? It would prevent opening the door from inside until the tire is moved from the outside.