I have a bolt-in 3pt from Corbeau and I just tuck it under my seat when I'm not using it. The rear strap stays outta the way of rear passengers pretty well. I was supposed to have gotten a quick release version so that the harness can be removed from the I-bolts quickly, and I reccomend that version of the harness.
I would think that the "latch" anchors in the backs of the rear seat would be a good place. That way there is a good angle for the upper straps. If you used a latch or hook, you could disconnect that part of the harness when not in use.
mine is like that minus the strap mounted towards the right (center). someone can sit in the back, but the strap might bother them, but belts in the back are still functional