Back in the '70's, there was a company (Segen, or something like that) that made external-frame packs out of all-natural materials. The frame was laminated wood, the bag was canvas, etc.
The hip belt was leather, lined with sheepskin. With use, the leather belt was said to conform to the contours of your hips. I didn't own one, but heard good reports. So using sheepskin has been done.

I'm guessing a different pack may be the answer. Try a bunch of them on with weight in them. I have bony hips, too, and have had sore hips with some backpacks.