"However, if you reversed the set-up and had the quick ejector strap attached to the plane, and attached it to somewhere on the harness, say the articulated hip ring, that might be more efficient"
If by "quick ejector" you mean a large version of an RSL snap, than that's exactly what I see in a lot of planes in Europe (mostly C208's). And it works very well, without the need to modify a harness. I don't know the legal implications, but apparently it's not a problem here. Getting such seatbelts in our C182 on the other hand appears to be a big problem to the authorities, unfortunately.