Seems like a harness-bar setup could easily be integrated to the MP4-12C interior by using a Cromoly (or the like) bar which spans from B-pillar to B-pillar, connecting at the stock seatbelt shoulder-mount hardpoints of the passenger cabin.
The shoulder harness-belts would be affixed to the harness bar, whereas the lap harness-belts would be affixed to the floor-mounted stock seatbelt lap-mount hardpoints of the passenger cabin.
For additional 5/6 point anti-sub harness-belts, a Cromoly (or the like) bar can be affixed underneath the lower-seat cushion, attached to the seat-base frame or seat-mount rail/bracket..
Just thinking outloud, from afar.