I've mixed (more than) a few live shows, many of which were recorded in some manner. One of the bands I work for regularly records almost every show they're at, so I'm pretty familiar with the whole thing. I've always told anyone wanting an audio feed to use a separate microphone, and I *strongly* discourage people from trying to use the FoH send from the board.

I figure anyone tour/venue that does this will have separate mics in the audience area (probably near FoH) fed into the console and mixed into a separate subgroup that's used only for recording equipment. At that point, you may as well skip the mixer altogether (unless you're starved for money and can't afford decent mic preamps) and go straight to your recording rig.

As cool as it sounds, you don't really want the mix from the board, you want the mix the FoH engineer is making for the audience, which is what's coming out of the speakers. This is quite aside from the fact that the board is *not* the last thing to process the audio signal...there are compressors, crossovers, etc that are also operating on the mix and the engineer has taken that into account, while anything taken straight off the board won't.