Peter,
If the car needs to go through engineering then you'd be best to stay with lap sash belts. In any case I believe the FIA rotary buckles do not meet the ADR requirements for prevention of accidental unbuckling.
Schroth make a retractable push button 4 point harness which is claimed to be ADR approved and may appease your engineer.
There are quite a few threads on this subject in the
OzClubbie forum.
Speaking from personal experience race harnesses are a pain in the bum in road cars...leave them for the track.
Cheers