Only trouble is.... I don't have an Ixus 65 to test this idea. 
Buy one, test the idea, sell it !
I do however have an Ixus 60, and an even better idea (its amazing what you can dream up while walking the dog)...
If I use TWO diodes, one of which already exists on the board, our input becomes +5V tolerant, since a +5V signal will drop by the forward voltage drop of the diode, typically 1.2V... so we drop our +5v to 3.8V. which in my book is close enough to 3.7V (the actual stated battery voltage of an Ixus is 3.7V, but fully charged, it will be slightly more). I suspect the difference of 0.1V will be of no consequence.
Better yet since in this setup I wire a single diode between the USB ID pin (using the diode anode) and the opposite side of the power switch to +V Batt. (using the diode cathode) this has two advantages. First, according to the Ixus 65 schematic this signal goes to an existing diode first, and through that to the pin on the power controller chip, (hence as explained above I only need this one more diode drop of 1.2V to make my input +5V tolerant).
The second advantage is that this pin would be easier to find, and larger, and thus easier to solder to than one of the pins on the voltage controller chip.
To find out which side of the switch to connect the cathode of the diode to, I can either
look at the diagram (page 10, Operations FPC Assy. for example), or for cameras where the diagram is not available, I can check for continuity (zero ohms) between one of the "Power On" switch pins and the + terminal in the battery box with the battery removed. If we don't see this continuity, we can assume that the model in question uses a different method of detecting the "Power on" switch (labeled in the case of the Ixus 65, the SW_POWER signal).
I need to consider one more factor, there is likely to be an existing signal on USB ID from the host. Therefore if we connect a usb lead between the camera and our PC, we need to ensure this mod is not going to stop us from being able to use the camera as before...
If we consider the USB spec...
Pin Name Cable color Description
1 VBUS Red +5 V
2 Dā White Data ā
3 D+ Green Data +
4 ID None Permits distinction of host connection from slave connection
ā¢ host: connected to Signal ground
ā¢ slave: not connected
5 GND Black Signal ground
... so at worst we will have the diode anode grounded, which should be of no consequence....
Ladies and gentlemen I think I will need to find time soon to break out the soldering iron and the magnifying glass.
(Unless anybody else is up for the challenge).
