I know that this is an older thread, but I just had a talk with an engineer touring a pretty well known band the other night, and he freely gave me some of his tips and tricks that he uses. Let me just say that these guys crush it night after night, and have one of the best live sounds in the jamband scene (IMHO). Anyway, here goes.
ORTF mics, stage lip, pointing back at the crowd. These microphone sources do not go to the FOH mix, but do go to the 4 track recording of the show which is available for purchase later through nugs. What he does is route most all of his channels to a bus, that he then uses as a sidechain for those audience mics, to duc them when the band is playing. As the song level decreases, or the song ends entirely, the audience source mics comes up in the mix to the recording.
It's funny coming across this thread on TS, because earlier today I had actually stumbled upon this very same engineers question on an AVID forum.
I understand that this may not be possible depending on the desk you are using (he is driving a Midas Pro9), but is very capable if you were to record in the box. I have used this in my recordings in a much smaller venue (16-32 tracks), using analogue mixers and A/D's to PT's and Logic, as well as digi-logue consoles and Studiolive mixers.
I do not run my audience mics through FOH to the PA, due to timing, reverb, feedback etc problems. I guess if you really needed to for some reason, then a little math might get you closer to a realtime record from them rather then the natural delay.
I hope this helps someone along hthe way. I know it did for me.
Cheers!
Christopher