The camera connector kit, when torn down, reveals that pins 25 and 27 are in use. These are 2 of the 3 USB data lines - the USB power lines are not in use. However, even if you attach +5v to USB Vin, and ground it via pin 1 or 2, it will still try to check the USB data lines to see what kind of charger is attached. Since the camera connector kit is attached, it won't work. Even if you hooked up the USB data lines to your charger, it would send the signal out in parallel to both the charger and the camera connector kit, and that could return some really... interesting results.
The absolute dirtiest hack I can think of that MIGHT make it work is to put a switch on the USB data lines connecting to the iPad Camera Connector kit, and turn them off. Then, attach the charger, with the correct data lines, so it starts charging, then flip the switch the the Camera connector kit is connected to the USB data lines again - even then, it might stop charging because of weird voltages across the data lines, or it may damage the iPad, charger, etc - we just don't know for sure what would happen.