Yes, those are good thoughts above, a little mixer would be the best. Of course, you'll probably have to fight a sound guy about doing something like this at a gig...they tend to dislike giving up control of the sound {smile}.
Another thought. I sometimes plug a SM 58 (or any dynamic mic) right into a distortion pedal and then into the mixer for a really effected sound. If you have this effect in mind for a few lines or a section, that may be a cooler approach - you could run two mic's on stage: One normal. One super-effected. Go mic, to adapter that changes XLR to quarter inch, to pedal, to main board for the sound. Again, a sound person at a gig is going to hate you for doing this!
Or you can plug a mic into a guitar amp, that sounds interesting sometimes...
good luck,
km