I have just had a chance to analyze the Ixus 60 schematic (I have a couple of ebay Ixus 60s and even a spare Ixus 60 motherboard, so I can afford some collateral damage when I get a chance to try this).
The modification I intend to try is very simple. I am going to put a link wire between pin 4 (USB ID) on the USB connector (on the top side of the motherboard), and test point FP1002 (on the reverse side of the motherboard), which is connected to SW_POWER, the signal from the power button (which is routed from the user controls FPC via the DC controller PCB to the motherboard). This is by far and away the simplest mod, as it is all on the one board.
Test point FP1002 looks easy to locate, relatively large and therefore should be easy to solder a connection to. PIn 4 on the motherboard USB connector looks a little more of a challenge. Routing the mod wire from the front to the back of the board looks the easiest part. A couple of strategically placed small bits of insulating tape should secure and protect it.
I will put the diode, needed to OR this signal with the external input, in my modified USB cable, this will allow me to experiment with different arrangements of components without having to break open the camera again.
I have even sourced a suitable USB connector (from the connector set of a cheap phone charger for the car) which has pins 1,4 and 5 populated, all in a very neat tiny case which can be opened up, modified and re-uesd. All I need now is the time to actually strip one of the cameras, and fix the mod wire.
This should give me an absolutely stock looking Ixus 60 with the magic ability to be switched on and off via a special cable attached to the USB port (the same magic cable will also be modded to be used to trigger the shutter via CHDK).
Photos of the build will follow, as and when I get the time to try it.
If this works, I may even try the mod to the RTC chip (I think a surface mount CMOS 4069 (wired dead bug style) would be sufficiently low power to use as 3.3v logic inverter to provide the necessary +3v3 to switch on the camera from the /alarm or /timer signal).
This bit of surgery, however looks a little more tricky. The RTC chip package and the CD4069BE are pretty darned small, and thus tricky to solder. The RTC chip (IC1002 in the Ixus diagram) is however conveniently close to, and on the same side of the board as, test point FP1002.
I would also need to write a bit of code to program the alarm and/or timer registers (based on the code that the Ixus 60 uses to set the time no doubt). More of a long term project I think.