Possibly a kickstarter to code CMYK into gimp would be a good idea? If the gimp team doesn't let you do it, it should be easy enough to fork it with a bit of financial backing from a kickstarter.
Gimp is just one free alternative, i guess you knew about its cmyk troubles and posted because of that. Why limit yourself to Gimp though? Check.out other alternatives. I posted some in another comment in this thread.
