Dale, Let's take this one step further. I have a hunter's lumbar pack similar to the one you have pictured in your post. It's NOT military in nature, but has a large rear pocket and two smaller pockets on each side. Even without the detachable shoulder straps it's over a pound. So, I have approx 1100 cu in of storage.
The next logical step is mentioned below. Really can't say if I saw or read about the following idea long ago and remembered it, or if I thought it up me-self. I'll opt for the first if it's a good idea, and the second if it's a dumb idea. [Note: I intended the order of my statements to be as written - self-denegrating, besides I don't think that anyone can claim uniqueness with an idea like this - someone probably did this hundreds of years ago too.]
I couple this pack with, get this,...
a pair of thigh or leg packs positioned on the outside of each thigh like a western style holster, but much larger. They are approx 900-1000 cu in each. Years ago I made this arrangement from some hunting day packs. These packs had their own hip-belt and shoulder straps, but I've attached them to the lumbar pack hip-belt. To the bottom of each pack webbing was attached to allow it to be afixed to my lower thighs, just above my knees - sort'a western gun-slinger style.
When scrambling or traversing some very steep rocky terrain, it makes one very mobile keeping all of the weight around the body's natural CG. One can lean out to grab branches or rocks, sometimes with just the toes locked in a crevice and NOT have all the weight of the pack acting on the back and upper back region making this manuever more difficult.
Nearly all of my gear is pretty handy - a nice feature. Have used it on warm, dry weekends before starting to go UL. Even back then, before going UL, I would just use a bivy and a poncho for rain gear (NOT pitched as a tarp - never occurred to me to sew tie-outs to the poncho for use as a tarp - obviously, not too bright on my part).
The major drawback is the weight of the system (it's NOT Lightweight) and it's difficult to stuff a colder weather synthetic sleeping bag into any of the three large pockets in the system. I also feel that sometimes, if I overstuff the leg packs, that they get in the way squeezing through tight spaces - can turn sideways, but the 900-1000 cu in large lumbar pocket is back there giving one a large "caboose". I probably should have used smaller packs.
If I could still sew a bit, I would probably see about retrofiting the entire system with lighter-off-the-shelf packs (never sewed well enough to make a complete system from scratch) by cannabalizing them and retrofiting the system.
I was surprised to see not that long ago that Cabelas had packs (both lumbar and leg packs) similar (though NOT identical) to the one's I made some years ago. I thought that it might be a dumb idea since I have taken so much flak from friends when I have used this gear. Overall, I liked it though, with one major exception (see below) and never really minded their ribbing.
Personally, I don't like a lot of weight on my thighs either - this is my major "crab" coupled with some bulk out to the sides of the thighs and no pocket large enough for a cooler weather synth. sleeping bag. Maybe other's will feel differently or have a way to make it work.
I might have to revisit this idea again now that all my gear is UL and also pay someone to make a new system out of silNylon, spinnaker, or cuben (if it's appropriate for a system like this).