You should be able to zoom during the video without using CHDK, but you will be limited to 4-5x digital zoom, which looks horrible.
If you are using CHDK, make sure the option is selected on the CHDK menu and before you start recording the video zoom in all the way, 12x, now start the recording, as soon as you started recording zoom all the way out, now you can use the optical zoom freely during the recording, it will look great most of the time, but be aware that when using optical zoom during the recording the lens won't update the focus automatically, that's a known limitation.
When you are done recording use the Canon built-in menu function to TRIM the beginning of the video when you had the zoom set at 12x.
