I've never done it with cmyk. but that would really give a washed out look it seems.
and like you say, interesting.
What I was talking about was found on russel browns website. I can't tell you exactly how he did it but go to his website and look for seeing in Black and White.
He had three layers. The top layer was a black and white conversion layer but it was really cool
because you could go back to the second layer and shift the hue in sat/hue and emulate any color
of filter as you would use over the lens of a camera to i.e. orange would make the blue sky very dark
gray etc. As you slide the control back and forth you can actually see the b&w effect it will have.
John M.
http://www.russellbrown.com/body.html