I'm certainly in the camp of those who do not have an adequate frame of reference to know how to address your medical-imposed needs.

Having stated my ignorance, I do have to wonder: would a trampoline-style suspension work for you? Deuter, Osprey, and others make several different models with such suspensions, and they aren't exorbitantly expensive. In this style pack, the "working" parts of the suspension, that transfer the load to your hips without making contact with your back. The only contract with your back would be the taut mesh backpanel, which spreads any pressure evenly across your back.

I have no idea how these would work for you, but if you're near an REI, they will definitely have several models of these (most reliably: Osprey Atmos and Exos series packs. Just be aware most REI folks I've encountered don't understand how to tweak the other components of the Atmos AG to get a good fit. They can usually get the Exos right, however.)