I've used a GPV4 for a few years now. My ridgerest foam pad rolls up to line the inside, which provides rigidity for the pack. The pack is designed for a z-rest pad which becomes the "stays" in those outside back pockets at the top and bottom, between the straps. I've never needed actually stays when loading within it's design limit.
If you have to have them, it shouldn't be too hard to sew in pockets. Might try some sail batten material from West Marine...stiff, thin, and light.