As reyalp posted, you can configure CHDK to automatically start your script when the camera is turned on. It's an option in the CHDK script menu. The script can put the camera into shooting mode and then take pictures each time a button is pressed. Note that when the script is running, all buttons are disabled. They will not perform their normal Canon function. So you probably also want to check a different button so that you can exit the script and stop shooting.
Your little script needs a loop added. As written, it will check the video button, shoot if it is pressed, and then exit.
A custom version of CHDK is one where the core CHDK software is changed to do something special. Making the video button function as the shutter button without using a script for example. It is a lot of work to do by someone with special skills so a script solution is usually better.
Update : you can also configure CHDK to automatically start a script when you enter <ALT> mode. So a single line script that just does a "shoot" should work each time you enter <ALT>. You'll have to press the button again to exit <ALT> of course.
