I've done this, but only when I had spot mics (as I could vary the reverb by instrument). For standard 4ch mixes, I EQ the aud/sbd feeds to highlight something missing in each and then use the audience source as the reverb effect and mix it that way. Case in point, I had a super dry sbd taped in a cave, so I added a db or two of bass in the board tape and added a touch of tube/transistor excitement to just the bass, then mixed. That allowed me to get more of an excited but very "present" and controlled bass without utilizing too much of the boom of the audience recording.