The forward-facing fold-ups from BLRS come undone with no tools - once you
fold it up fully it slides off its mountings - when folded down (in use) it
has cunningness which stops it doing this.  they're nicely made in fact.

If you hunt in the group you'll find my post about seat belt mounts - it's
in a thread about selling a 110 - or if not feel free to email me.
However... You're not gonna avoid seatbelt things sticking up in some
fashion or another.  The seat belts in the middle row seats in the disco are
on clip-on mounts on the floor, I notice, so you could use them; you'd need
the mount that bolts to the floor, and if just fitted to a plain LR floor
they's stick up - the disco has an thick rubberoid under-carpet thing with
suitable holes in it so the belt mounts are recessed.  Also the inertia reel
bits stay put, it's only the extra bit that the belt plugs into that's
removeable (without tools).

an Inertia reel one will need to bits of mounting - one under the floor (or
more likely the wheelbox) and a second L-shaped one to bolt onto that and
carry the reel, which is designed to fit on a vertical surface.  A suitable
section of heavy angle (say 5mm thick) would do that.

3-point upper mounting will require quite a bit of thought, assuming you're
not prepared to have it fitted through the side of the vehicle.  I expect
you'd have to have a suitable braced thing which attaches to the floor (it
can share the inertia reel's mount on the floor) and goes up to the
roof/side joint.  it'd then need bracing forward or backward, I expect.
Hmmmm.  wonder how you get seat belts tested, I could end up making sucha
gadget and marketing it, if I can find out how to get it approved.