ISO override is a little bit different in its effects than AV or TV. Since CHDK allows you to override TV to be shorter or longer than Canon firmware allows you and AV to be (in general) higher, the ISO is rather not possible to be overriden beyond Canon firmware highest or lowest berrier.
Nevertheless in this case, since at 1.3 s the Canon firmware block ISO at 100, which is not the lowes or the highest possible ISO at all on this camera we can believe that you can override it.
The only way to be sure of that is just to try;) So take a few shoots with and without different overriden ISO values and check wheter overrides make any change to the picture in brightness and noise. I'm pretty sure, that this will work!
