Don't know if this is within the realm of what you're looking to do production-wise but I used one of these mounted to my Suzuki 650 . . . shouldn't be much of a stretch to mount on a mountain bike: http://gopro.com/
I think the OP is talking about making a shoulder rig out of a set of handlebars. When I mentioned the ergonomics, I started thinking about how you would do it...I assume you're thinking about using the bar ends over your shoulders? I think the curl of the bars would be too tight; they would likely position the middle of the bars right against your chest, so although they may work as a component to a bigger assembly attached to the bars themselves, I think it would be pretty confining and uncomfortable. I've recently made a couple of DIY shoulder braces out of PVC pipe and have been really happy with the results. Comfortable, light, highly configurable to your camera/body shape, and cheap. That may be a better option