If your concern is the old factory belts , Why not just replace the OEM unit with an aftermarket 5 point set and only use the lap belt ?? That will give you as good if not better safety than the original one and not mess with your car .
I have thought about doing as you are suggesting but I still want some sort of upper body restraint. I'm going to see if I can get a custom belt made that would include a 2 or 3" belt that would attach to the B-post OEM belt mounting point similar to OEM belt set-up
