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!