Ditto rharv- That's what I do, a one man show-I sometimes am lucky enough to get a female freind of ours to join in occasionaly, so what i've done is "tweak" all the songs to include (or exclude) parts that she does, save the files and use those when she's available. (I also do this with a "scaled back light show" for smaller places) yep, once the "main" file is completed, it's relatively easy to do tweaks for special occasions.
And for Silvertones- you're right-you'll probably have to do the on-off thing for it to work for you. The beauty of the Digitech is that you can just turn on-off the harmony generator without affecting other parameters of the unit.