OK... so this thread got me to sit down and try and throw together some extended load lifters for my Mainframe. It has been fine for me carrying gear for a couple nights of hunting, but I wanted to see if I could adapt it for some heavy load carrying. Haven't tested it out yet with weight, but thought I would post some pics before this thread got too old.
The modification is easily removable, didn't cost anything as I had all the buckles and scraps of webbing laying around. The ridged bit is an old carbon fibre tent pole I had laying around that I cut in half - seems pretty stiff.
This idea was stolen from a guy on Rokslide that did the same thing.
Hope the images work, if not I will try and figure it out: