Thanks for the replies so far, everyone. Just some brief replies to some of your comments:
(1) Yes, I had begun to realise that my needs fall into whatever is the opposite of a "sweet spot," due to the timing of changes in technology (towards disc brakes, away from 26", and towards tubeless).
(2) Thanks for the link to the old thread, PH. I had search for tubeless in the forum, but the thread title of that thread hadn't pulled me towards it.
(3) The main thing drawing me towards tubeless is that for the first time a year ago, I switched to tubeless on another bike and have lost count of the number of times I've been with groups who have had to stop for punctures and I've not had to has really impressed me. But I've managed to get through several decades fixing punctures in inner tubes that it won't kill me if I don't find anything.
(4) I followed the link in the older thread to the Velocity Cliffhangers, but it looks at the moment like they no loner sell rim brake compatible in 26". I've emailed them and will post what I find.
(5) Have found one wheelset that looks like it may work - disc brake compatible, but with a machined rim that will accept rim brakes. It's a set of Pro-Lite Sport MD-10s, built in Taiwan. They are pretty inexpensive (< £150 for a set), which makes me wonder about the quality, but I'm looking into it further.
Will post updates and eventual outcome for anyone who finds this thread at a later date.