Thats what i did with my track car - just welded up a frame out of angleiron to bolt the race seats to. I also ditched the stock seatbelts for harnesses. However, most people tend to have brackets tho with holes drilled in them which give you a small amount of adjustment - like tilt and height etc, so maybe that counts?
Im hoping the new regs will be like the current MOT and very dependent up on the actual testers. My R1 motorbike for example has a loud race exhaust on it, no headlamps, no indicators and a few other race bits, but because my MOT man is understanding it always passes. Nothing illegal, its just how the regulations are interpreted. For example, with the exhaust, it says something like "must not be louder than equivalent motorbike" or something like that, and my man's view on this is that "well all these sports bikes have loud exhausts so its fine". The lack of headlamps and indicators is also fine as for motorbikes you can get a "day light" mot which is primarily aimed at motocross bikes, but theres no law which says you cant apply it to a sports motorbike.