Override really really overrides. CHDK obeys you totally and only overrides what you tell it to override. Canon's firmware doesn't know CHDK is going to override until it's already shooting, so ISO will remain the same regardless of the override.
Tv override is not a Tv mode for cameras that don't have one (are there any, is yours one?). To some extent it probably could be made to behave like that, though, you can search through the feature requests section. Usually Tv override is used either to exceed artificial Tv limits (to set faster or slower shutter speeds than Canon firmware allows) or simultaneously with Av override and manual ISO (or ISO override) to achieve M mode like behavior on the many cameras that don't have an M mode.
I'm not sure what you're excepting and what you know, but you could study the exposure value system on e.g. Wikipedia... your camera probably has Ev compensation to come to your rescue unless you're overriding too far (i.e. more than your camera's ISO range can adjust for). Also note that Ev compensation can be scripted to +-4 Ev even though your camera's menus only show +-2 Ev.
