interesting lifting & do you
there are plenty of series landies with rear door mounted spares that aint got buggered doors/hinges. If you fit an extra hinge in the centre and an extra striker plate thingy (there are special ones for rear door mounted wheels) on the inside then it should be fine. Theres a '74 series 3 in my mates drive. One of the least buggered bits is the rear door! (with wheel and mount)
The hinges wear out anyway eventually but the replacement balls and pins cost less than a quid when i last bought any!