It occurs to me that the poster of this topic may have missed
this previous post on the same subject... This obviously only applies if you are trying to control the camera and it is running from an external power source.
Most (all?) Canon models will power up (and also load CHDK and any script you want, if you set it to start at power up) if you hold down the power button (i.e. tape it down or keep it pressed by some mechanical means), prior to switching on the external power source.
In other words, the answer may be to power the camera from an external source, which you control (with a relay or transistor/mosfet or other switch).
This certainly works with the following Ixus 30,40,50,60,70 and 850 IS.
I would imagine it works with most others. All of these models will power on, and stay on, despite the fact that the power switch is held down.
If however you need to control the camera directly and run it from its own batteries, I fear "surgery" is the only way.
Once the camera is on, it would be the responsibility of your script to take the picture, and if necessary stow the lens afterwards, before the external power is switched off again. A clever feedback method for switching off the external source might be to use one of the camera LEDs as half of an optoisolator. A blink of the LED could then instruct the power controller to switch the camera back off once its work was done.