Coming back after a week of "how on Earth am I going to get the nozzle properly cleared", I was surprised when I homed Z and the right side got stuck when I tried to move down to take out the bolt. Not sure what had happened, but the right side had gone higher than the left and was wedged, unable to move by motor power. Turned the motors off and manually screwing it down again helped, and after re-levelling it seems to work. But now I'll have to keep an eye on whether the right-hand threaded rod mount has gotten loose/worn.
With that repaired (for now), I got out the hobbed bolt and cleaned the stripped PLA off it. (Mistyped that as "striped PLA", doesn't look like anybody has tried that:) Also used a trimmed-down Q-tip and then an appropriately-sized hex screw to attempt to clear any flakes and bits out of the holes. Then I heated the loose hotend up to 230C, lifting one end to prevent backflow, and did a "hanging atomic". Without the extruder gears in the way, this one did indeed make that satisfying "plop" and showed dirt on the end:
The second one was nicer, didn't show any signs of dirt:
After that, I used the metal wire to try to push stuck dirt out, again at 230C and did one more atomic. There's a little bit of metal visible in there, so more cleaning is needed:
At the next one, I could see something black coming out with the filament I pressed through.
I rather like the sheer photographic look of these. They're very odd.
I noticed the jump up in temperature just before getting down to 90C, so that's apparently a thing the firmware does, not a result of me jiggling the wires.
Probably sticking the wire in from the tip was counter-productive, coming from the top and eventually pulling it out through the tip would prevent dirt from being pushed back into the tube.
After this showed nothing, I put everything together again and did a test cube - now on shiny new Kapton tape. Which didn't stick at all. I should have checked beforehand, Kapton is apparently not that useful for PLA.
It occurs to me that if, as Tim Hatch commented, the hobbed bolt was really poor quality, then maybe the hotend is as well. In which case, attempts at cleaning it are probably rather futile.