I have had a go at this today. some trouble...
I did not want to drill in the chassis memebrs. I thought I could put a magnet under carpet. However - in terms of location it does not work so well with Euro door panels as they have the pockets going close to seat when you close the door. If you want to place it closer to pillar A, to be able to reach it with closed door, th ebelt is too short when adjusted (and I am not slimmest person in the world). Also: if the magnet is, say 5mm thick (what I had at hand) you see it, carpet is not lying nice and even. When a magnet would be thin, say 1mm, passenger may not use it correctly, as it will be invisible...
Magnet on the carpet - issues with holding it on to the carpet. It goes with the buckle. Tried carpet screws, tried velcro, does not work. You could screw it to the carpet with washers underneath, but it is giving it too much depth.
What I ended up with is attaching a magnet to the buckle. Then when you put down the buckle on the door sill, it stays there in order. You basically do not notice the magnet is there. The only point was that I had a pair of neodyme magnets 40mmx10mmx5mm. 5mm is a bit too high to blend in in the buckle. I will order 40mmx10mmx1 or 2mm - that should be fine. To make sure it stays in place on the buckle I used a thin double sided adhesive tape.
When I replace the magnets with 1 or 2mm thick ones, I will check if it works in practice, I will post pictures.
Maybe there are other ideas -please provide.