While possible and worth investigating, keep in mind that Canon uses & has used a multitude of different audio chips. Some are capable of greater things than others, and disabling AGC/ALC will be one of the things that may not be possible on all hardware. But at least previously in the vxworks days firmwares had clear text strings hinting which audio chip the camera (probably) had.
So, one could start by checking which audio chip the sx20 fw refers to (if any) and search for that string on other firmwares to find suitable targets.
