I've supplied outs from my rig for front of house a few times, but mostly with an engineer I knew well with whom I worked on a weekly acoustic show. He enjoyed being able to use the signal from my very nice preamps and bypassing the ones in his elderly Mackie board.
As far as a big rock and roll show, I doubt you'd ever get away with it.
Some venues do have an extra split already wired in (The Fox Theater here in Boulder does), but I found that
Audiopile Pro Audio* Welcome to Audiopile have a very reasonably priced 24 channel split. I've been using it for a couple years and it's held up very well. Not isolated, but I've never had a problem.
Edwin