I can think of 2 options.

1/ Get a set of rear belts out of a later MPi model.  These can be
mounted in the rear pocket, with the bolt through the rear wheel arch
(with a spreader plate under the arch).  You will have to make a
"wedge" to go under the inertia wheel so that it mounts verticle on the
angled arch.  Appart from the mounting hole through the arch they use
the 3 existing mounting points that you'll already have.

2/ Here in Oz, when later Minis are imported they must be fitted with
rear inertia belts to comply to the Australian standards.  My 94 Cooper
has aftermarket units that are available from most big Auto Accessory
stores (usually you have to order them though).  These mount flat on
the rear parcel shelf close to the outter edges - with spreader plates
onder the shelf.  These use the bottom 2 existing mounts, and the mount
on the rear pillar is not used.  Problems with fitting these is you'll
probably have to remove the fuel tank to allow you to mount the one
above the fuel tank, and the spreader plates limit the speaker options
if you're into noisy things :-)

A couple of other things to remember is that any belts you fit other
than the standard ones should be engineered - not only for legal
reasons but also for your own safety.

Also when sourcing the inertia reel units, make sure you get the
correct ones for the angle that you are going to mount them on (ie the
ones designed to mount vertically in the front of a Mini won't work
properly if you mount them flat on the rear parcel shelf.

HTH   John

Your biggest challenge would be that they'd have to be mounted pretty
much vertically (just as they are in the bins) or they won't work
properly. You certainly couldn't get away with using the the stock
mounting point for the fixed 3 point belts. I would say that your best
bet (other than fixed 3 pointers) would be rear inertias from a late
model Mini and mount them in the same way (to the inside of the wheel
arch). Remember to put a reinforcing plate on the other side of the
arch so they don't pull through in an accident.