Since we don't have any scientific studies to rely on so far on this thread, the alternative is to look at the question logically.
If you look at picture of a human skeleton, its seems logical that weight transferred by pressure on the shoulders and back will compress discs in the lower back. On the other hand, weight transferred by pressure on the iliac crests (hip bones) appears to be transferred directly to the legs through the femoral head (hip to leg connection).
If that is correct, it follows that the greater the percentage of weight resting on the iliac crests, the less pressure on the discs.
AFAIK, a frameless pack cannot accomplish this, due to the amount of weight transferred through the shoulders and back. While it seems counterintuitive, a pack will also transfer weight through the back, because we tend to lean forward a bit when backpacking. So a frameless pack would have to be extremely light to give an edge over a framed one that transfers more weight through the iliac crests.
The only commercially available pack I ever saw that transferred most of the weight through the crests was Jack Stephenson's Jackpack sold by WarmLite. The center of the hipbelt was attached to the bottom center of the frame, but did not touch the center of the back due to foam on the belt to the right and left of the center attachment. The belt ran from the center, over the iliac crests on each side, and out to the front points of sidearms. WarmLite's pack should not be confused with the other sidearm packs that had no rear center connection between frame and belt, so transferred the weight to a point in front of the crests, causing the pack to pull back against the shoulders; thus defeating the whole purpose of the frame by putting weight back on the shoulders. Unfortunately, the WarmLite packs were discontinued.
As has been suggested in several of the posts, that leaves you with finding the lightest frame pack that will put the most weight on the crests, and less on the back. This is fairly easy to tell, because such a pack will put less weight on the shoulders, which is easily felt. But as was also pointed out, this is not helpful if the hipbelt slides down off the crests after walking a while.
One approach is to weight the packs when trying them out and see what happens with different packs, and with different lengths if available on the same pack. One problem here seems to be that when more weight is carried, the pack slips down, with the front center of the belt pulling on your gut, and the straps pulling on your shoulders. Sometimes a stiffer belt will help to prevent this slipping and pulling. I've tried lots of packs in the stores that feel great, but when weights are placed in them, slip badly. If you can find ones that hold firmly in place on your hips when weighted, thus taking more weight off your shoulders, the lightest of those should be the one for you.
Except for one thing. Even if there is no slippage, if the weight transfer is more though a lumbar pad pulled tight against the center back, that might not do much for your discs either.
It is also nice to find a pack that allows the hips to rotate freely when walking, without your hips having to alternately lift the pack as you step, a process which drains away a lot of energy. Someone mentioned the North Face Back Magic on a thread here recently, but those were discontinued also. Nevertheless, a number of pack makers have tried to address this, and you can feel how well they succeeded by walking in the store with a weighted pack.
None of the above makes much sense without strong leg and back muscles. Fortunately, that is something we can address with exercise.