I'll answer the various questions above.
I agree to what you say about the half-shoot solution. It does work but causes interface issues. ALT is used in many places. 
I'm thinking either making long-menu or long-display enter the ALT mode. I'll check out the other cameras and see if any have no-print buttons.
As for the questions of time lines... I think intial work will be a couple to three more weeks (provided life doesn't get in the way). After that it's hard to say. This is my first port and I imagine bugs persist for a while.
As for the firmware it is very young and not worth testing with at all. It was simply a proof that I'm actually working on it. LOL I'll post a newer firmware tommorow that actually does stuff....
Note that some cameras let you choose your alt button, search for CAM_ADJUSTABLE_ALT_BUTTON in the code.
One idea I had is using the play button. You could have a long press do the original function. Requiring a long press to get to alt mode is more annoying IMO, because you frequently want to get into alt and quickly change something.
My reasoning (based on using the SD990, I'm assuming the ixus 100 works the same way) is: In play mode, the canon behavior is also weird and mostly useless. It shuts down only if the lens is already retracted. In record mode, holding down the button slightly longer to go to play wouldn't be a big deal.
Downside, this could be very confusing for new users.
