When working with designs meant for screen printing, the original artwork was done in RGB, then a team would separate the color channels (in Photoshop), one channel per ink to be used. They could technically do CMYK directly, but it didn't look good for a wide variety of purposes -- you can imagine a flat-filled cartoon character would be pretty much impossible. It would look a bit like comic book halftoning, probably. The shop would use that when they wanted to print Thomas Kincaide-esque sweatshirts for grannies. They would also use additional channels for things that weren't colors, like adhesive (for foil, usually) and clear inks.
I don't imagine that having more than three or four color channels is a new thing, or difficult to deal with. I would imagine even the prosumer technology would allow you to choose between various rendering intents. Probably the color separation is handled at the driver or device level, but TIFF, PDF, and DCS 2.0 (??) should handle extra channels natively.
A few more details on screen printing for those who might care: The actual screen printing process was not computer-controlled as a rule. The smaller shops I worked at printed a transparency which was transferred onto the screen by a photographic process, but the large one had a computer-controlled airjet "printer" that would knock out the design. Usually they would do a few samples by hand, to work out what ink and screen combination to use (different mesh sizes and ink thicknesses produce slightly different effects), and adjust finer details like when you would "flash" the shirt. That is, hitting it with a very high powered xenon lamp for a few seconds to dry the ink, before applying a new layer. You could do some interesting painterly effects with wet-on-wet ink; you can also make a hell of a mess that way. Flashing also tends to affect the color somewhat, especially for temperature-sensitive inks. After you get a few good samples, you send them off to the client as a proof. Then you would set up your automatic press for a run of a couple hundred. Color balance was something that the press operator kept an eye on after that point. After printing, the shirts are sent through a 400 degree open oven on a conveyor belt, for perhaps 10-20 seconds, to cure the ink.
Very fun job, the ink is messy as hell. I would still be doing it, but working with computers pays better.