So I finally put code into Gimp-print to provide a new input mode that gives full control over all physical channels. This is like CMYK, only more so (if you have a 6 or 7 color printer). Somebody with a spectrophotometer can make really good profiles and use all six colors optimally.

I don't have time to write more about this right now, but if anyone wants to play with it, it's on the "generic_color_branch" in our CVS.