Some time ago I purchased a Schroth racing harness off eBay on a best offer. The rear of the harness at the shoulders had a female seat belt clip similar to the sort of factory clip that attaches to the seat. Included was an inertia harness reel that was designed to be bolted into the rear of the vehicle and the reel pulled out and clipped into the back of the harness like a regular seat belt. So the harness was attached to an inertia reel like a regular seat belt and not a fixed point.
I noticed at the time that the male clip of the inertia reel was almost identical to the regular seat belt clip, so I tried just pulling the rear seat belt forward and clipping it into the Schroth Harness. Well it fit like a glove. I never used it like that but given that it isn't designed to have a great deal of movement before it locks up in an impact it looked as if it would work well. I KNOW that it's a God part and obviously tugging on the connection with your hands is hardly a suitable method to test it wouldn't separate in a crash, but assuming it is the correct fit and as secure as the stock belt it would make a damn neat solution. Factory belt mounts that obviously are safe, mounting point above shoulder level, inertia reel and simply disconnect like the stock belt.