The audio on this camera is uncompressed 16-bit stereo PCM at 44KHz, and I wasn't thinking of using the on-board mic!
I was wondering if maybe you recorded a continuous tone at a low-ish level on one channel, and your live sound on another, then the AGC would react to the live sound and affect both equally. Later you could use the tone level variations as a guide in automating a fix to the level in the live sound. It really would work like Dolby B!
But if you can now switch AGC off with software 2.0.4 my query is academic...
Cheers for answering!