Here's another idea, less expensive than a total custom pack - take a mass-produced pack, and take it apart - the suspension is pretty much just attached to the back of the pack. Remove the rest of the packbag and sew on a new one, really simple in Dyneema. Probably save you a couple of pounds and you could do it yourself if you are handy or get PennyS to do it for you:
http://specialtyoutdoors.com/
Or you could just remove portions of the existing packbag to simplify it.
If you find that an external frame would suit your needs, you could do it for less money, since you can buy some frames without packbags. Probably a little heavier, though.