...However I don't know where to put the diff. Where do such things go?
You can upload it to the CHDK Bugtracker
, it's not only for bugs, also for diff's & patches, language files, docus, scripts, tools etc., files up to 2MB can be uploaded.
btw - on the Ixus 860 you can switch between playback and record mode with the PLAY button, so a keyboard hack should be possible.