I don't think that it's physically possible, due to the CCD limitations. It might be possible using a CMOS sensor.
Exposures shorter than 1/30 are obviously possible, because camera is using shorter exposures on good light (at least my A620 does) - it's evident from EXIF of thm files
Also, watching LCD in photo mode on very low light shows that the camera is using exposures longer than 1/30 to generate preview
It would be fine if we could control exposure in video mode, like in camcorders (CCD and CMOS), but CHDK options are only for photo mode by now