O.k., this is what should have been done first:
The last idea on the list- making a toggle from plastic bits- seemed the best one. It's like a legal tag, not one of my favourite bits of stationery, but a tried and tested fastener. Also, it uses up stuff I've horded and doesn't require buying anything at all! Win-win, as they say.
The toggle is made from the little tube from the squirter bit in a deoderant bottle, pierced by a thumb tack then trimmed down to size. The plastic tube is easy to punch through and the hole made is easy to get a threaded needle through. Win-win, again!
The thread shank is made by pulling the thread through the fabric, then the hole punched in the tube, tieing it around the toggle square knot-style so there's a shank length, going back into the fabric, holding the toggle so both lengths of the shank are the same and doing a little back stitch knot. Easy. A normal hole punch at the top of the cardboard spool to attach it with. Easy. Not sure if it's a win-win, because the aesthetics come into play here, and this prototype does look rough.
I like where this is going. Needs a bit of measuring, and I'll need to find the right fabric to use. Not sure if I want to pretty up the toggle or not. I could try putting beads on each end, running the tread through the tube to give it a bit more colour, but I kind of like the way it looks now, a bit rough and functional, not fussy. I feel like it's enough to be getting on with.