I know it is possible to use a script, and I have - It allows zooming and does the refocusing on cameras that don't have it built in (S2, S3, others?).
However, there are 3 nagging issues (for me):
1. Need to load the script.
2. No "autorepeat" (can't hold the zoom lever down for the zoom to go further).
3. Nasty noise in the recording.
I think a solution is possible in code to:
1. turn off sound recording before zooming.
2. Keep zooming in case zoom lever not released.
3. Focus after (and return sound recording)
Since the Digital zooming in video is not so hot, I guess we could simply do away with it, and have CHDK use real zooming while in video recording.