This is an idea that's probably occurred to nearly everyone whose ever worn a harness, but when we saw a post on the O & P listserv about an upper extremity harness for a patient with axillary pressure issues it seemed like a good time to get this out there. The basic idea is a harness sewn into a stretchy, wicking t-shirt like an under armour shirt. The idea has since been prototyped with duct tape (try it by threading duct tape through the back ring and substituting it for the loop by passing one piece over the shoulder and another around the torso), and it lasted for a couple of days. The beauty of this is that in addition to being more comfortable you could have several and throw them in the wash more frequently then you might an entire harness.
The current prototype includes a minimal lower harness on the arm with a loop to keep the backplate (just some stiffener sewed between webbing to support the T-bar) from flopping. The fact that the harness is sewn into the shirt helps.
Update April 2008:
Since the duct tape prototype, Advanced Arm sewed some 2 inch webbing into a shirt which works pretty well (well enough so it's all I'm wearing -Jon).
Update August 2008:
We have since made another iteration, and it works great! The key is having the webbing meet low enough down the side and around the front so that friction keeps it from riding up the armpit. I want more of them so I can treat them like T-shirts. The friction lock works well after we placed it on the shirt instead of the arm so that the weight of pull was distributed across a loop instead of a single piece of webbing. Sometimes you learn your basic physics all over again.
