Ok, so I soldered the wire from the via you pointed out to the pin on the header, but no joy. When I press the pwer button I get blank screen, and the green light around the eject button just keeps flashing. If I press the eject button the xbox reboots and the same thing happens.
If I try to boot the xbox with the chip disabled (hold power button) the led on the xecuter switch flashes red briefly and then the xbox reboots to the problem above.
If I press and hold the power button the xbox boots then switches off and then switches on again with the xecuter led red and the eject light flash red and green.
So either the point you identified is not an alternative LAD0 or I've really fried the xbox now.
Let me know if you have any other alternatives that I can try. I gonna disconnect all the wires and see if the xbox will boot as normal then.