Maybe not the solution you are looking for but I think that it is a good one...
Aarn will have out in early spring their new photo pockets.
They will attach to all of their regular sized packs (not the day packs)
The idea is to have the weight of your photo gear (not the tripod...) at the front to balance the pack weight at the back. With this system you can also access the gear as you walk. They are waterproof as well.
I use the Aarns for heavier than about 23 or so pounds, at that point the pockets really do make a big difference.
Here are the specs :
Fit on all models except Marathon & Mountain Magics
* Divided for camera body and 3 additional lenses.
* Separate waterproof Dri-liners for camera and each lens
* Separate zip entry for top and bottom compartments
* Foam stiffened for stability.
* 4 mesh exterior pockets, 1 map pocket
* Converts to a daypack with add-on shoulder straps
* One torso length fits all
* 500D cordura, 210D ripstop nylon
* Grey/black

Volume 14L + 2L / pair
Weight 470gm / pair?

( I suggested the photo pockets to Aarn Tate a couple of months ago, he sent me back the photo ans specs of the finished product, all within a few hours. He was ahead of me by a few months)
Franco


see packs here :http://www.aarnusa.com/
(more detailed info at http://www.aarnpacks.com/news/index.html)