Wow. Lotsa good info in this thread... I'm looking for a harness bar and harnesses simply to keep me planted. I'm sick of sliding around in the seat...
Please don't go that route, its not safe for the street or track. You can't change one part of a safety system without it affecting the other components.
Using a harness bar and a less than 5pt harness for autoX is acceptable IMO but there is still a risk involved.