As far as I know, that would require to replace canons auto-iso-algorithms. I remember that a1ex did some work on that a few months ago, but had to surrender.
The idea was brilliant. The user could even adjust how fast the ISO should adapt to exposure changes, something that isn't possible right now.
But A1ex result had a few glitches, it had jumps in the exposure at times. Again: afaik.
Therefore, the feature has been removed again.
