(image pulled and modified from step 6 of the iFixit MLB replacement guide for this model)
I believe the power pads for this model are located at the top of the logic board, just to the right of it's top left mounting point. The two flat silver pads are larger than similar ones in the area. A T8 should work for shorting them.
Something to keep in mind is that a damaged or faulty keyboard could prevent a Mac from starting if the power button is always registering a press. If I were troubleshooting this I would first try to short it using the power pads. If that didn't work after a couple of attempts (the flat ones are more difficult to short) I would disconnect the keyboard's flex connection (near the RAM slots) and attempt it again.