UPDATE:
The chest strap was a response to a problem with slippage; when I arrived at my final design, I found that the shoulder straps slipped off my shoulders sometimes when I raised the camera to my eyes to take a picture. I solved the slippage problem by adding the chest strap. Slippage was never a problem in the rest position, or when walking, only when I raised the camera to take a pic.
None of my early prototypes had the chest strap, and slippage was never a problem with them until the final design. After all the complaints on the boards about the chest strap, I went back to the drawing board and re-evaluated the design. I tried the harness on a few other people, and found that slippage was not a design flaw, but was caused by the fact that I had adjusted the shoulder straps too long, and there was too much slack in them.
Once I snugged the shoulder straps, the slippage problem went away, and the chest strap became unnecessary.
I haven't tested the concept on rough terrain like a mountain hike, but on regular terrain it's steady as a rock. As long as straps are adjusted snugly so that the camera sits on your stomach, just above the belt line, you can walk around all day and never worry about the slippage problem. I've worn my prototypes to WDW, to Hershey Park, to the National Mall and its various monuments and museums, and on a couple of paid photo shoots. Not only did they work fabulous, but they never drew a second glance or mention from anyone; I had actually been a little worried that people might laugh at me when I wore it.
So, with the removal of the chest strap, I believe I have finalized my design, and I've even had my first order! But even so, I'm not quitting my day job just yet...