There's no restriction on seats, Mat. You can basically put in whatever you want. If the seat does not come with proper openings for the belts to pass through, however, then the seat must have bolstering to prevent sideways motion. And no, you can't put holes in the seat yourself.........
Having been in rallies in both street seats and race seats, though, believe me, you will be MUCH happier after the rally is over if you were sitting in a properly padded racing seat. Your body takes a pounding in a rally, and a race seat lessens the motion considerably.
It's like the difference between a good night's rest and a night having slept "wrong", you know what I mean?
The lap belt and crotch straps should not pass over the sides of the seat, but through the seat in order to wrap and hold the pelvic region over the greatest possible surface.
The lap straps must fit tightly in the bend between the pelvic crest and the upper thigh. Under no circumstances may they be worn over the region of the abdomen.
Holes may be made in the seat if this proves to be necessary in order to avoid such an occurrence. Care must be taken that the straps cannot be damaged through chafing against sharp edges.
I just checked the online rule book. You are correct. I stand corrected (well, actually I'm sitting right now.......)
It used to be that you couldn't put holes in the seat, because if the belt was bent at an angle through the hole in the seat, then the seat could rip further in an accident, which would loosen the belt and allow the competitor further movement forward or sideways.
Okay, so I would now say that, yes, you can put a hole in your seat. I just wouldn't recommend it for the above reason.