Your idea is creative, but I think you need to be cautious about this. I have used similar rappelling harnesses for helicopter hoist training. Even for a healthy dog they can be uncomfortable or even cause injury if not designed and fitted properly. For a GSD with a back injury I suspect lifting in a harness could easily exacerbate the problem.
If you decide to pursue this idea, I suggest at least looking into a commercial harness that uses a spreader bar to distribute the weight more evenly. For example:
They seem to be aware of the types injuries that can be caused by lifting, so perhaps giving them a call would be worthwhile. Hope this helps, take care.