I have been looking into retractable seatbelts for my sedan ( coupe seats fitted)
I can get aftermarket belts but they fowl on the reclining mechanism and my worry was that belts would not be running vertically.
If you are fitting inertia belts to the rear of a coupe you may have to fit a bar between the inner rear pillars- I would check with your local seat suppliers as they would know.
hope this helps