Off the top of my head, I'd guess that would have to be a one-off harness design and not something even close to DOT approved restraints? If so, he needs someone who can create
the best (safe while comfortable) design which might be better than adapting
a racing harness.
One good option may be a master parachute rigger who makes or repairs parachute harnesses. If this makes sense, I'd suggest he go talk to the folks at http://www.skydivemonterey.com/
in Marina. They can hook him up with a creative rigger who will comfortably hook him up.