I've done exactly that, but now that you've made me really think about it, I don't know if it's a good idea.
I need to check the schematics of the mackie boards, but if you have the XLR attached to the mic, and the 1/4" attached to a guitar, and the guitar volume is turned up, you'll probably get the sound of both, so make sure the guitar volume is turned all the way down.