[edited to correct some findings - wrong version]

ISO override is working for me with no flash - otherwise the same as asm1989 reports - I have brought it down to 60 ISO ok and pushed up too, with Av and Tv settings constant its changing the exposure broadly inline with what I expected though I haven't compared to an DSLR yet.
Forcing flash (any value) doesn't work - camera does not crash though.
Single shots and bracketing with Tv works ok with no flash (at the moment that doesn't bother me - as I am using a movie light).
Side note on camera crashes : given the camera has an iris the ND filter isn't needed as far as I can see - I tried it out though to see what creative effects I could get. With ND filter IN and aperature open the camera always crashes.
I have found long Tv expsoures when bracketing causes a crash. However it is not always reproduceable.
Sometimes with a bracketed sequence I seem to get a different exposure in the first shot when compared to a single (non-bracketed) shot with the same Av, Tv and ISO. It does not happen often and I am still trying to reproduce it everytime but so far no luck.
Bracketing only works for Tv - in Av mode for example with an Av override set and a Tv value it is produces brakceted shots as expected with changes to the Tv values. When set to bracket Av's I noted in play back it shows the changing F stop value but the image is the same. I have a manually set of brackets where just the Av is changed against a constant Tv - the CHDK brackets show no difference.
Minor point - the CHDK battery meter always shows 100% so have to use the Canon meter - camera has run out of power and the CHDK meter is still showing full.
Thanks for your work on this - I am happy with its current functionality with manual Av and Tv values set for single shots and Tv brackets is a bonus - gets me out of a problem with the underwater housing.
Will added these findings to GitHub.