well then it’s really up to the musician and his understanding of the tools. If someone decides to scream into a mic while everyone else is singing quietly there isn’t much you can do other than slam the fader down.

Maybe I am missing something but it seems like just some careful attention to gain staging and making sure all improvised sections stay within a certain threshold of amplification will solve the issue. Using a digital box like the finalizer might be more suited to critical studio listening situations than in live performance situations.

check out the stuff by these guys

http://www.fmraudio.com/products.html

small units and proven to be very useful live and in the studio