Choose ALT-mode button

[emlyn]

All buttons are already used, so whatever is chosen as the alt-mode button will conflict with something.

Another option would be to have two buttons that must be pressed together, but I have found this difficult to use because unless both buttons are pressed exactly at the same time, the first function will already be activated before switching to alt mode.

Also look into user-selectable "ALT" button
See http://chdk.wikia.com/wiki/Adding_support_for_a_new_camera (search for CAM_ADJUSTABLE_ALT_BUTTON)

[emlyn]

Another option would be to differentiate between a long and a short press on a button (e.g. DISP), using one for the native function, and the other for the ALT mode.
I don't know if this has already been implemented on another camera, or how difficult it would be...

[emlyn]

Seems it already exists to use a short click for alt-mode, long click for original function, search for CAM_EMUL_KEYPRESS_DELAY and CAM_EMUL_KEYPRESS_DURATION, also see http://chdk.setepontos.com/index.php?topic=1687.0 (point 7)