I have a thought on the short military packs (that is designed to be used on top of a webbing). Couldn't you feasibly use a bumbag (like the snugpack response pack) or similar to carry the burden? That would also give you a handy day pack. I saw a weight weeny kind of guy doing this and he swore by it. Altough he used a back pack in thin sil-nylon packed with really light stuff. I once saw an osprey bumbag that had better waist belt than I ever saw on a ruck sack (bar some models from klättermusen).
Just a thought.
Just a thought.