Anyone have an idea to where it'd be possible to mount buckets and harnesses while still being able to get to the back seat in an R33 GTST?
You can but its not a best practice. The best position for a harness strap is the bolt holes for the rear seat belts. I have a guide on how to install a 4 point harness somewhere I will post it upno is does not, you can get three point ones and bolt them to the seat and the bolt that holds the back seats down.
Could you post or PM me a few pictures of that?