Without knowing what i am talking about, i would day porting CHDK to the HV20 will be probably a challenging task.
If Canon would provide a firmware upgrade, it would be probably possible to decipher it and to disassemble to check if an CHDK port is possible. I never heard of an firmware upgrade for the hv20, so without this firmware upgrade things are more complicate.
Second possibility is to find an other Canon camcorder with the same processor. I think the Canon XL-HL1 uses the same DIGIC DV II. Canon did provide a firmware upgrade for this one, some people have a SD card with the firmware upgrade file on it. But the file is not available for download anywhere. It might be possible to analyse this firmware to write a blinker for the HV20 (think, thats how most CHDK ports start...). With available firmware it might then be possible to port CHDK to the HV20, but this still could be difficult.
Third possibility is to hope for some luck, investigate how the HV20 reacts to things which work on Canon digital cameras. Maybe digic II is not so different. E.g. ver.req ... is there a firmware update menu item, e.g. if you place a ".fir" file on the sd card... With a lot of work, it then might be possible to write a blinker. If you are really serious (and more the hardware guy) you might want to read out the firmware
directly from the flash chips