It turns out that the ceramic-tip plotter pens don’t come apart at the top of the flange as I expected. Instead, there’s a snug-fitting plug with a tapered top and an invisible joint at the end of the body tube:
Refilling a pair of defunct black ceramic pens didn’t bring them back to life: an ample supply of fresh black ink never made it from the fluff to the nib. Soaking the nibs + fiber shafts in 10% ethanol for a day created an unappetizing black vodka shot that did nothing to get the ink where it needed to be.
The right time to refill those pens would have been, oh, probably a decade or two ago…
Some stuff, you just gotta throw out!
The Smell of Molten Projects in the Morning 
You could try looking for Rapido-Eze pen cleaner at your local art store. A small container is about $4, it’s designed for this sort of task, and has been known to work wonders for cleaning old hardened ink out of technical pens.
I have no idea what kind of ink might be inside the plotter pens, but the solvent can’t possibly make the dried-up ones any worse: thanks for the suggestion!
If this isn’t the door into the Eighth Circle of Dweebdom, it’s close enough for me… [grin]
Yes, when the remaining option is to give up and throw something away, all sorts of things become worth a try for people like us.