I'm using a Canon SX210 IS with CHDK. I know that capturing long exposition photos is a "battery draining" process, but it could be better. Why should the LCD's backlight be on while taking the photo? It might be a bug of CHDK. Otherwise, does anybody know how to turn the backlight off??
Generally, you press the DISP button to cycle through the various display options, one of which is off.
No experience with CHDK, but that's how regular Canons work.
Editing much later to add: I have since installed CHDK on my S90, and the DISP button works the same as with the stock firmware. So simply pressing it should cycle you through basic display, extended info display, and display off.