OK, now it's time to come clean, George! You're probably on first-name terms with every REI or Eastern Mountain Sports clerk within a 100-mile radius. They have a checkout line just for you and personal greet-and-guide shoppers to meet you at the door. Somewhere, some ex-pack salesperson is bobbing on a 75-foot yacht bought with the commissions earned from your purchases.
And, I hate to say it, but I'm not sure there is any gear that will help. We share a similar build and, like you, I have basically no hips. I, too, also find that heavy packs tend to slide down, particularly when wearing Gore-Tex bibs or similar clothing. The solution? None that I am aware of. Aside from just toughing it out, about the best you can do is shorten the pack torso a little, so the hipbelt rides slightly higher than might normally be comfortable. That will give it a bit more purchase.
A real fix, of course, would entail some sort of fabric applied to the clothing that adheres to the material lining the hipbelt. Gregory packs, among others, try to address their half of the equation with a high-friction fabric called "Gription." I suppose you could have an outdoor gear-repair company laminate something to the bibs, offering a little extra friction. Cordura, a very heavy fabric, has a lot of inherent stickiness, and is available by the yard so might be a good candidate for that. Be our guinea piggive it a try and tell us what happens.