Design-wise, I give you an "A" for originality. That is not a concept that I have seen previously. Plenty of owners hang plenty of stuff off their rear barn doors, but it's all mass-market prefab, such that it looks like van tumors (in some cases extreme). In sizing and shaping the box to be respectful of the vehicle's form, it yields a more streamlined and thoughtful appearance.
Engineering-wise, I don't feel qualified to comment. I've spent years going round and round on what the rear doors should or should not accept weight-wise, and how they should accept it.
I am fairly convinced that the sheet metal alone cannot hold much - there have been too many anecdotal reports of deformation, reports by owners who tried this or that in the way of installations. Every time I see a ladder bolted into the sheet metal alone, I cringe. There's a reason why Aluminess installs them in the gutters
Clearly the hinges are more robust, as spare tires are approved for hanging there. And those suckers weigh 70 pounds each, plus mounting hardware. They are not "spare tires" at all - they are wheels. Rim plus tire.
Anyway, this is interesting. Thanks for posting.