All you really need to do to turn the comp on is figure out which two pins on that header are the power switch pins, and touch a screwdriver to them.
Sounds like something you don't wanna normally do, but it's the same as pushing the power button, and if you are gonna config bios to power on upon power restoration, it's all you need.
(if it were me I'd still end up with a little tiny button inside the case somewhere though, connected to those pins, cuz holding a screwdriver there for 4 seconds to force power off would be a pain)