If you look at the roadster seats on an IM, they have the shoulder belt attached to the upper edge of the seat in a similar manner but not exactly like that classic car seat.
While, it may not be high enough for all body styles, I mean human 
if you fit the seat it is a better system. Now for those IM's that are roadsters and go to the UK they have a high shoulder belt arm welded on the frame to meet the seat belt regulations for the 3rd point.
On my recaro seats, a much higher back rest seat, my shoulder belt goes on top of the seat just like the roadster seat and it is slightly above my shoulders so it does the job but may not be officially high enough by standards.