The wire is very hard stuff, so I would try to source one perhaps from a quick-release skewer nut if it can be made to fit.

It would be easy to file a couple of flats on the selector ring, and I recall seeing index shifters that were thus machined from the factory.

Perhaps a circlip plier could be made to engage the holes??? I would lube the shifter up first, then perhaps ply the shop mechanic with beer to get him/her to have a go at this(?). There is usually a hole in the back side of the shifter that is the best entry for some light lubricant (might make the ring easier to turn).

If cosmetics are of no concern, I would grip the ring on the mounted shifter with a pair of Vice-Grips, pushing in hard with the tool head-first to effect some purchase through wedging-action. This often works on various types of tapered-head, tamper-proof fasteners. Don't slip and ding you downtube though!!!!!!!!!!!!