The big issue with fixing harnesses to rear seat belt points is that in a front end crash, you risk badly damaging your spine because you, the seat and the harness can/do pivot forwards & down (think what would happen if a flimsy fibreglass seat back snapped!). MSA (Blue Book, Part K) say that the rear harness straps are prefered at an angle of +/- 10 degrees from horizontal, down to a max of 45 degrees. So your car would be unlikely to pass scrutineering if you mount to the rear seat belt points anyway, and you won't get to a nice shallow angle without a harness bar. A poorly fitted harness will be a lot more damaging than a standard seatbelt!