“Our torso lengths are 24" and 25", respectively.”

Since you want external frame, this guy advertises “• The frame telescopes 9 inches to automatically fit any torso length. Compresses to airline carry-on legal 20x14x8.” http://www.luxurylite.com/packindex.html I think it’s the lightest external frame out there.

However, there are plenty of other lightweight packs out there (<2lbs) and <$150 that fit comfortably howbeit, they are internal frame. The Gossamer Gear packs (24” torso) http://www.gossamergear.com/cgi-bin/gossamergear/Miniposa.html lets you use convoluted foam for the back (and framework) which helps in promoting airflow.

The heavier the pack and load, the more you’re going to sweat. So by lightening the load and getting a smaller volume pack --- minimizes the back sweat marks.

Good luck in your pack choice!

-Barry