Yes but on your A570, where (memory location) did you monitor the override ISO value ?
Also, where (memory location) did you monitor the actual gain value ?
I am not seeing a Canon-set gain value unless I set one.
I am setting 0x105070+4 to a gain value and calling 0xFFB5B740 with no parameters.
I am running out of time, tomorrow I will test the Panasonic FZ38 I bought some time ago for this application.
There are many 'creative' modes for movies, you can set shutter speed from 1/30 to 1/20,000 sec (!).
Also, there is the option of MJPEG encoding which will give better quality for extracted frames.