The flexsteel seats bolt to the floor, nothing else. Obviously they have a very rigid frame which transfers all the force to the floor. I think this is why the company won't sell them aftermarket.
Now that I think of it, the shoulder straps on my Yukon XL come out of the seat back and the only attachment is the seat to the floor.
Regarding fitting normal shoulder harness to the standard seats - at least one member here did it. Newkid64 I think. He attached the harness to the main rib that runs right behind the the seat. There used to be pictures of this on this site but I couldn't find them the last time I looked.
If you want others to be happy, practice compassion.
If you want to be happy, practice compassion - The Dalai Lama
1984 310 Limited Motorhome
Courtesy Parking (W/S/E/Wi-Fi) on I-5 in Northern California, 70 miles from Oregon border