I went in today to do some measurements on my thermistor. After trying with an infrared meter (which didn't work at all) and with an ohmmeter (which didn't work while connected to the Melzi, so was hard to control), I realized I had gotten not a B57861S0104F040, but a B57861S010_3_F040, which is a 10K rather than 100K thermistor. I should have double-checked when only one value was available at Conrad, but I thought that was because I had given sufficiently precise search terms.
There is a setting for a 10K thermistor, but with the comment "!! do not use it for a hotend. It gives bad resolution at high temp. !!". That doesn't sound good. I would definitely not be able to do ABS with that, not sure if it would be good enough for PLA even. But it'll be as easy to swap out then as now, so I'll do some first experiments, knowing this is a potential issue, and see how it goes.
Change the setting and uploaded new firmware. Tried setting to 185º (PLA), but for some reason Pronterface thought 476º was much more exciting and kept switching to that as limit. The heating went fast, and while I tried resetting to lower and lower temperatures, it kept switching back up. WTF? Looks like the limit is confused by the 10K setting - it also starts out at -35º. I guess it's going to be tricky at best to get it to work anywhere close to reliably. Bother again.
I can get a new one for €2,50, but with €5 shipping. I might as well grab a few extra then, for leaner times.