I did my power on/off mod on my controller for my xbox using a controller from a company called venom it cost me approx £12.99 or $18.00 ,who's controller includes two extra push buttons used for "turbo" & "Clear" as they call it.
any way after a little detective work with the multi meter it turns out that this button can be easily used to control the power on/off, by tracing its "output" side along the circuit board till you find its termination point and as it turned out this controllers termination point was a wire going to another small board with a chip on it so i desoldered this wire as i wasn't going to find a use for a "turbo" button, and soldered it to the yellow controller wire that i had also desoldered. and i now had a "earth" leading back to the xbox, as that is all that pressing the on/off button on the xbox does( It provides/removes an earth as required).
Now inide the xbox you need to snip the yellow wire that is going to the circuit board on controller 1 (i snipped all 4 yellow wires from each port this way i can switch it on/off from any port) then i soldered them all together with a extension wire going to the circuit board that pokes through the metal sheilding on the face edge of the xbox this is the small board that is used for the on/off function you then solder the extension wire to the number 8 pin number 1 pin being the top one and you are done,this way it looks like it was meant to be there in the first place and as the buttons on the controller are clear i painted it red like an on/off button it looks cute.
and as there i another button i can use on the controller i may change the connectors and used a cable with extra wires in it to ones like they use on a pc joystick, so that when i fit my xbox in a custom case i can use the other button as a eject button on the controller.
if anyone needs any help just pm me ok