When I was doing an independant study in color printing at a lab that did primarily Ilfochromes, their Ilford rep got exited when he heard they had a few of us learing the process and got us a ton of literature, among that was some instructions for spotting. IIRC (it's been a little while and I didn't read that part too carefully) the basic idea was to bleach the dyes in an area, and use color spoting dyes. We were taught to just reprint anything with dust though, and as far as I know that's what the lab did for their clients' work as well. If you've got access to anyone who could get you the literature from Ilford (acctually Ilford would probably send it to you directly), they've got some great information about everything from the chemistry, to masking to retouching. If you can't get ahold of it and would like to, PM me and I'll see if I can find it and scan it for you.

Peter