[javanix]

It would probably be technically "possible" but I doubt that you'd end up with anything that sounds listenable.

The only bootleg audio that usually sounds decent are soundboard recordings, and when you find audience tapes spliced in with them the difference is immediately noticable.

[kokey]

I think theoretically you can make it quite good, but it's not going to be a simple task and you probably can't rely on purely the fans' recording. I think the best way would be to modulate them together based on a weighting depending on position or distance away from where the music is coming from, also discarding distorted recordings etc. That said, you probably only want this lightly mixed in along with a good recording from the desk, all adjusted for the viewing angle.

[gridspy]

Perhaps bands who want to be popular on this site could provide concert recordings to splice in.