I was actually thinking about making my own hinges, Outlaw. Since the surfaces are curved, the lines of rotation wouldnt line up if you put more than one hinge on. You have the same problem with the shoulders if you put hinges on flush with the cut points. Im still not sure how I would tackle that problem... but here's some nice visual aid:
The red is the line of rotation. Notice that the natural shoulder hinge rotations dont line up. This would want to 'split' the front in two when they opened, or warp the shoulders inward... cracking your fiberglass, or not moving at all, even though there are hinges. I have the stomach line in the ideal place. In that position, the plate can rotate almost 70 degrees out from the chest. The green lines show where I would attatch mounting points, which can easily be made out of metal. The hinge rod could be made out of 1/4" all-thread, using nylon nuts to lock it in place and keep it centered. Also, weights could be added to the bottom half of the floating face so that it 'hangs' correctly. Or, one could physically strap it to the under armor.
Sorry if this is getting off-topic. If this needs to all go in a seperate thread, someone say so; I just havent gotten to this stage in my own building so I havent had the need to post it as 'progress' yet. Maybe we could all get together and make a 'detailed chest build' tutorial?