I’m posting this in the interest of posterity after a conversation with my local printer repair guy and a Facebook dialog with Walker.
According to my local Epson repair guy, the Achilles heel of the R3000 (and 3800 and 3880) printers is the the black-ink switching part of the print head. As they get lots of use, the Glossy/Matte ink switching mechanism can get jammed and stop working altogether and kill the K channel. The best fix is replacing the print head, but that is costly - he estimated about $600 all in. Which makes no sense when I get an Epson refurbished R3000 for $500 if I lurk on the Epson site frequently enough.
What Walker suggested is to repurpose the unused Y channel (normally the K7 lightest gray channel in regular Peizography) to the K channel. I have to modify a shitload of the .quad files to copy the K values to the Y channel and then zero out the K channel values. But once I did that, it seems to be back on track.
So instead of sending the printer to the graveyard, I will probably be able to get some more mileage out of it.