I thought about making a spare go up with the hatch for years. But the problem is that the hatch is not up to supporting it so you would have to make it hinge on a bar that goes across the roof bolted to the gutters on each side. But because now the hinge point is different to the hatch it has to be released from the hatch before both can be raised as the lengths change as it raises between them. But you could make the arms holding the spare telescopic themselves as they raise or have it the same as the Bestop heavy duty TJ carrier where they have an extra set pivots in the arms which allow for the change in length. There was some good pictures of it in Off-Road Adventure issue November 2004 which I have scanner a picture of below and you can sort of work it out in their installation guide as well.
http://www.bestop.com/installation_guides/61950.pdf Step 13, 23 & 25 show the links in the open mode where they are shortened. When closed they sit flat and inline with the rest of the arms.
If you had the support bar that goes across the roof sitting up a bit you could mount a bracket to it with a really short gas strut from there to the arms to help lift it. You would still have some assistance from the ones in the hatch and maybe even change those for higher pressure ones to help as well.
I have scanned a page I was talking about.
Now imagine this vertically and you can see what I was thinking about. The small arms move away from the body when opening it to take up the difference in length due to the different hinge points and then lay flat against it when closed.
I think this could be the answer to the problem and it would not be that hard to buy one of these and modify it to fit an XJ but better off making from scratch if not time poor.
Just need to work in the gas struts ( like used on some wheel carriers ) to help assist the lift and centre the tyre fixing.
What do you think??