I think it's definitely possible. The pack I use for most backpacking (homemade) weighs 16 oz and carries about 30 lbs comfortably.It has aluminum stays, full padded hipbelt, etc. If I were making my own pack for really big loads. I have no doubt I could get it under 4 lbs. the key is to simplify the bag so that most of the weight of the pack is suspension. A simple 7000 cubic inch sack of 4 oz dyneema can be as light as 8 or 10 ounces - that's just a sack, no pockets, no zippers, no suspension. So then you have room to attach that sack to the best suspension you can get. That kind of simplicity is the key to keeping the total weight down. Pockets and compartments and zippers ans flaps are heavy. Just remember that features=weight and you're on your way to a light pack. All that said, I suspect I would be thinking pretty hard about going to an external frame for a load that big - depending, of course, on what you're planning to do with that behemoth on your back. But the same rules would apply - keep the sack really simple, not much more than a sack attached to the frame. Most packs on the market, McHale's included, have lots of pockets, flaps and zippers, and all that stuff adds up fast. If you talk to Dan and make it a challenge, I bet he'd love the idea of making a superlight pack for really big loads.
I would also be asking myself how much volume I really need. If you carry lightweight gear and then lots of food, the food is very dense comparatively, so the total volume you need might be less than you'd think.for instance, I got 26 lbs of food into 2 bear canisters totalling 1530 cubic inches. If I carried 80 lbs of food at a similar density (which is about how much food I'd imagine I'd have out of a 100 lb load), that would be about 4700 cubic inches. I'm sure I wouldn't need another 2300 for gear if it's lightweight gear. your gear my be different, though, since we don't know what type of trip you have in mind (climbing or whatever). Anyway, it's worth careful consideration of just how big the pack really needs to be to get the job done.