I've long been intrigued by the idea of building a pack on top of an existing internal frame. It could be something UL, or simply a pack exactly like you want it.
Since most of the lightweight/standard packs I've had were made by Granite Gear I've mostly thought about doing so with a Granite Gear Nimbus or Stratus frame. No separate stays and framesheet- just a molded carbon fiber with attachments points for the hipbelt and shoulder harness. To build a pack around this frame you basically build a sack with a sleeve with the correct dimensions to hold the frame- you could build something airy and ventilated with only a small amount of 3D foam padding; or you could retrofit a heavy canvas portage pack to make some sort of 10 lb beast.
The weights of the Nimbus packs (40-64 oz) are higher than a lot of folks here use, but I still wonder what could be done with the GG framesheet, a simpler hipbelt, and a lighter shoulder harness.
For those who aren't familiar, here's a photo of a Granite Gear's Tepex frame:
I'm going to remove the frame from my Nimbus Meridian tomorrow and get weights on the frame, hipbelt, and shoulder harness. I'm a sucker for minimalistic utility and modular systems, so I'll have to give it a try at some point! :)
Just wondering if anyone has done this and what their results were!