So, a few have asked so here goes...
I have a Sabelt 3" web 4 point camlock installed in my P10 for track day use.
It snaps in and out so it's out of the way daily driving...
Here's where it anchors:
Behind the fold-down seat behind the driver, I have an eye-bolt through the sheetmetal bulkhead backed by a 2" x 5" or so 1/4" steel plate.
It's drilled through a double thick part in the sheet near a crease so I think it's suitable for anchoring the rear leg. BTW, note that the rear leg is the anti-sub portion of the 5 point harness attached to the pair of shoulder fly-backs using some heavy duty washers as a link.
The outboard mount is another eyebolt drilled through the floor and backed by another piece of 1/4" thick steel.
The inboard mount is a piece of 3/16" thick by 1" steel bar, drilled and bent and anchored through an extended bolt that ties down the seat rail. The bolt is through, then the strap, then a backing nut.
Sure, it wouldn't pass SCCA tech, but it's strong enough for my purposes. BTW, with the B Pillar where it is in the G, I don't think that a harness and no cage is an issue. Especially if the stock 3 point belts pre-tensioners fire, I'll bet that you have as much "side-duck" as in the harness. I'll take my chances. Besides, I'm that much more likely to lose control if I'm in the stock belts sliding around on the leather than if I'm firmly cinched down and not fighting to stay put!