its a cool idea, but it would be tough to make it work. you cold build a frame from angle steel on the back door and bolt everything to that, but your still going to lose as much or more vision in your rear view as you would with a spare carrier. that and the hatch would be a HEAVY S.O.B. to pick up.
look into building a swing out carrier into your stock rear bumper cover, theres a few threads on it and it always turns out looking really clean.
98 ZJ- 4.0L, NP242, 12" kenwood sub, Has a DVD player!!! bone stock for now- Hers.
98 XJ- 4.0L, NP231, AW4, chryco 8.25, mid arm 3-link, 285/75 Herc terra tracs on moabs- MINE!!!