I haven't spent much time with it yet, but I did try using ISO Override and found something worth mentioning (for CHDK Beta4 with firmware 100c). The feature does work, but the actual ISO used doesn't match the value selected.
For example, I selected ISO 400 (4 with a Value Factor of 100). ISO 400 was shown on the display, but after taking a picture it seems that ISO 640 was actually used. When ISO 800 was selected, the actual value used was 1250.
I don't know whether this bug is specific to the SX260 or is seen on other models too.