At the risk of displaying my total ignorance about your condition, two other things might substitute for bending stays. If you can consistently keep your total load below 20 pounds, a frameless pack like the Granite Gear Virga might work: you can fold an inflatable sleeping pad to fit down the back, like a backpad, then inflate it slightly for a semi-rigid "frame" that automatically molds perfectly to your back.

If your load is normally 20-25 pounds, Granite Gear's Vapor Trail might work. It has a semi-rigid framesheet without stays (I think), covered with about 3/4 inch of foam backpad that covers your entire back and, again, molds nicely to your whole back automatically. (The pad is big enough to extend a 3/4 length Thermarest Prolite Plus pad into a full-length pad, simultaneously lightening your load by a half pound and allowing you to store the empty pack tidily out of the way under your lower legs.

I've used both successfully for a number of years, and the comfort level is incredibly high for both packs, as long as you stay within the weight limits. That may mean going very light or even ultralight - but I'm assuming less weight would help with the back problem anyhow?

Again, if I've totally missed the effects of your condition, I apologixe.