I think I read somewhere that a direct box or something like that could help get rid of feedback. I might remember wrong though.. Anyone know anything about that?
That could help, but you won´t be able to hear the distorted voice. It is only possible to divide the signal into two and use the clean for monitoring and the distorted for audience. But anyway you will have problems with feedback even from the main speakers if you want it to be really distorted.
Live, it's going to be the same issue that you'll always face - maximizing the sound source signal relative to the mains output by proper gain staging, mic posititioning, and speaker positioning. The higher gains you're using, the more tricky it will get to make it all work, but keep in mind that you may not need nearly as much gain as you might think to make the effect work.
depending on the type of music and if it will work with it, you could just turn off the stomp box when there's no vocals also just tweak with the gain and volume and see if that'll work for ya