I'm not aware of this being done since there are some pretty obvious issues to deal with. The hatch hinges are not designed to carry the extra weight and dynamic forces produced by the tire and mounting assembly, the gas struts will need to be much stronger, the hatch sheet metal would require solid reinforcement, the release mechanism is in the way and the rear window, the up and down mechanism and motor all make reinforcing very tight on space.
If you can engineer a good way of making it work and dealing with the issues I mentioned, you'll be on a winner, since there are many of us (including me) who would love to get the spare out from under the vehicle.
I'm not sure if you are aware of the swing out tire carriers that were on Hilux Surfs but not US 4Runners, but if you can get one of those it would be a simpler job to install then making a hatch bracket.
