There was an overlap between having a B20 and having children...and I had eye bolts between the seat back and cushion. These were mainly for harnesses for the front seats, but I was able to hook a lap belt between left and right to secure a baby seat.
If I was doing it again I might have a "purist" seat cushion and a second one with cutouts for the eye bolts. The bolts go in and out in a few seconds. That would also be a "child friendly cushion" with a washable cover or wipe clean.
It might be a bit of a tangle getting everyone in and out but a line of eyebolts would permit various combinations of rear belts, an anchor for the top strap "conventional" three point belts, three or four point harnesses depending on the trip / event. Front lap belt fixings are fairly obvious. If having work done then beefing up the areas where the bolts will go from the underside is an idea rather than having reinforcement plates on the top surfaces.
If wanting more than lap belts in the back it gets more tricky. My car had an aluminium firewall sealed and riveted to the seat back and under the parcel shelf and we pondered beefing up that structure to support seat belt mounts, but didn't. Another option we considered was bridging between the roll cage legs. Depends if the shoulder straps are more for show (to reassure granny...) or you're wanting them to hold a 15 stone adult in a big frontender (as racing).
I doubt you're going to have a rear cage, but while on the topic the diagonal of mine was cut and brackets made so it could be bolted in or removed in a couple of mins. I never quite got round to making special padding for the brackets but would wrap something round and give stern warnings. I was going to say it was used for a wedding like that, but just remembered the groom had to slither in behind the diagonal as the wedding was before that mod. I took the passenger seat out for the bride to have leg and dress room. They really are very versatile cars...I really don't see why these mini MPVs have so caught on
