Some 0.3mm metal wire was hiding in the sewing room. As a first test, I heated the hotend to 220C and inserted wire, goes right in. But that of course pushed molten plastic back into where it shouldn't be. Probably not so smart. So instead I cool it down again and dismount just the hotend (made a little tricky by the fan mount being in the way, it could benefit from being a little lower). Spookily, while pulling out the hotend, an M5 bolt with washers came out as well. That was unexpected.
I found a hex wrench (new sets again useful) that's about the right size for the upper hole (which for reference is 3.5mm). So now I heat up the hotend again while it's hanging in mid-air, being very careful to only grip the hotend with well-insulated pliers. The upper part is quite cool, though.
It's dripping black gunk off the side. Ew.
I was able to get the metal wire in from the top and pull a piece through. Only a little PLA came out with it. Then, after letting it cool down a bit more, I realized that was not the best approach. I should leave the wire in while cooling down and pull it at about 90C - essentially a wired Atomic. Didn't do much.
Tried another two regular Atomics. Didn't see much in the way of black stuff, but the filament seems to go in more easily now, and comes out straight.
Finally doing a test cube:
And we're back in business!
So to clean:
1) Remove nozzle from extruder, but leave it wired.
2) Use 0.3 mm metal thread from the top to pull gunk out
3) Clean wider part with a hex key
4) Reattach nozzle and do an atomic or two.