Good timing on this thread. I've been wondering the same thing, although I'm multitrack recording and mixing in post, so I do the level riding at home in post.
I've seen lots of bigger bands using shotgun mics very near the stacks pointed outward, and I believe they generally use inverted phase. In particular Allman Brothers, Gov't Mule etc, who will be selling CDs after the show, or muletracks, etc. I assume when the music is blaring they pick up mostly stacks, when it's not they pick up crowd. I was going to try this a couple of weeks ago at a good sized stage, but I didn't have the extra tracks.
And reverse polarity is just a XLR cable with pins (2&3??) swapped on one end.
Another issue becomes "the nature of the audience noise". If there is some awesome solo and the crowd goes wild, I'll pump that up a little even though the music is already blasting. If the band goes into a 60 second tuning/swap instruments break between songs, there is probably cheering at the end of the song, then it becomes undersirable background chatter as people get bored. We want the former, not the latter. All of this is hard to automate, and suggests that level riding is inevitable.