Ah, on 5D3 I was able to re-implement ISO 100 from Canon ISO 200. That is, I'm using a signal twice as hot in the internal electronics (where most of the noise comes from). The signal is pre-amplified by 1 stop before leaving the CMOS and I've turned down the post-CMOS amps to get the same amount of highlights as with ISO 100 (but with CMOS amp set as if it were for ISO 200).
For ISO 66 I can no longer do this trick (I can't get from Canon ISO 200 down to ISO 66, it stops at just a little above 100).
On 5D2/50D I don't know how to do this trick; the registers are different. On 60D I know how to do it, and I believe most newer cameras will be similar. 550D... don't remember, but I believe it's more like 60D.