The method using a carabiner does work, and a little research says its load capacity that way around is about twice the breaking strain required of the webbing of the day - though I think the requirements for webbing are a fair bit higher now. However, I decided there's a still better option.
The static belts come with an adjuster that joins two bits of webbing together. One bit has a sewn loop holding it to the adjuster and an eye to bolt to the attachment point on the inner sill behind the seat at the other end. The other bit is threaded over the locking bar inside the adjuster, with one side going through the plate at the end of the extension that goes to the buckle and then up to the top of the B post, and the end of the other side is more or less loose, just riveted into a plastic bit that holds it to the part of the belt that goes over the lap.
The adjuster is shown left of the middle of the lower run of webbing in the above and has "RAISE TO ADJUST" on it. The static belt shown here seems to be a later one than that I've disassembled and shown later, as it has a more complicated plate at the belt end of the extension with two holes, where the older one has only one hole, like a Platypus.
So, I thought, I can replace the section in the static belt that is threaded through the adjuster and goes up to the top of the B post, with the webbing that comes from the inertia reel - having that run up to the hanger at the top of the B post as normal -, then down through the loop on the extension, and threaded through the adjuster from the static belt. All that needed was that I had to (again) unstitch the eye that bolts behind the inertia reel, so I can thread that end of the belt round the locking bar in the adjuster from the static belt. The bit of the static belt that's already stitched onto the adjuster then goes down to where it did on the sill, but now the inertia reel itself is bolted over it.
The only problem in that is that the eye on the end of the static belt that bolts to the sill is not quite the same as the equivalent one that normally fits behind the inertia reel - it's shorter and the hole is smaller. I could have unstitched the one that's on and restitched the belt over the longer, larger hole eye instead - and that's still an option. But rather than that, I decided to use a spare stand-off from the hanger at the top of the B post, and fit that between the inertia reel and the sill with the smaller hole, shorter eye on that. That stand-off is a little longer and thinner than the original, but as it's good enough at the hanger, I assume it should also be good enough at the back of the reel. And I could shorten it a little, till the eye is almost trapped by the carpet - they aren't that hard to find if I get that wrong. I haven't riveted the plastic bit on the end of the belt where the inertia reel belt eye was, but that't only cosmetic. Also, that end has been unstitched and restitched several times now, and its a bit tatty, so I'm thinking of trimming the tatty part off. But then, this is the worst of all the belts and extension bits I have, used just as a proof of concept.
So now I could fit any suitable inertia reel, i.e. that bolts that way up on the sill and has the right width webbing, joined to the lower part of the static belt. That should include modern Securon belts, for example.
The issue for those very concerned with originality is that, as well as it not being the original coarse webbing MikeyB described, there is the adjuster at the driver's right hip that serves no purpose - other than it joins the webbing with an "approved" sticker on it, though one from when God were a lad - and the plastic bit tying off the free end.
That one is with up on the left if it matters.
Graham