If you are already using and arduino, couldn't you the use it as a HID device and send the keyboard (not sure if it does mouse as well) directly without going through a PS/2 emulation?
Otherwise, that would work fine, but not all motherboards support power-on via USB HID, most support PS/2 though.
Most Uno clones skip the atmega16u2 and use a cheap USB to serial adapter instead, which will not let you reprogram it as a USB HID