But if only black is a special case, why didn't .5 .5 .5 get undercolor added? Photoshop turns #808080 into 52C 43M 43Y 8K.
I have sent the printer files that previously worked and I agree that it's essentially their problem. But they're making some noises that I should be sending CMYK files. These EPSes are generated on a server and my tests using GhostScript to apply a color profile and convert to CMYK haven't produced good results.