Roger: you may want to consider the Journeyman backpack from Kinesis, which may be viewed at
I have been using it since last year, and am very pleased with its versatility. The medium front-loading module fits a 503/w A12, PME45 prism and 50 mm. lens perfectly with room for an extra lens, shade for the 50, and a Pentax digital spot meter. The module also has a zippered top pocket that can accomodate a light meter and a five pack of film. The remaining interior space of the pack is more than adequate to fit hiking gear that one would take on a minimalist two-three day hike; and there are plenty of lash points on the exterior to accomodate other gear. A tripod can be lashed to either side; or can also be center mounted (there is a pouch available that is designed specifically to accomodate that need perfectly).
As an alternative, you can also use the remaining space to add other modules to expand the pack's photo gear capabilities. If you ask nicely, the owner of the business -- Richard Stum -- will modify the pack/modules to fit your needs (within reason). I had him modify the bottom edge of the module insert to enable fitting different lens pouches upright so I could adjust the interior configuration to suit different needs. For ex&le: I use Kinesis' single and twin lens pouches under the main module to fit a third lens (capable of up to a 250) in the former, and a spare A12, a lens hood and a variety of filters in the latter. There is sufficient space left over (1/4 of the interior) for carrying a basic survival kit, a change of clothing, the stock column for a Gitzo 1228, and a small ground sheet.
Another beauty of the outfit is that it is modular, and many of the pouches/bags that you can use inside the Journeyman pack (apart from the main front and top loading modules) are designed specifically to be attached to Kinesis' system belt (or carried with an optional shoulder strap). The kit is very well made (quality is on par with mountaineering equipment) and reasonably priced -- the backpack, harness, belt and front-loading module would cost roughly 3/4 of the National Geographic pack that you are considering.