Yes, keyboard power on can only work on the legacy keyboard port. This is because keeping the USB port alive enough to manage a keystroke is substantially more difficult than on ye olde keyboard controller.
Keyboard power on is the only reason why my keyboard is still plugged into the PS/2 port. I'm sacrificing two USB ports on the keyboard just for this, but I really like turning on the computer with a simple press of a key, much like on a Mac!
Your PSU must have enough +5VStandby juice to keep your keyboard powered. 2 Amperes or so.
Mind you, the computer consumes 10 to 15 watts while waiting for the keypress all day. Add what all the peripherals consume on standby (monitor up to 30 watts, printer, whatever else), and you'll see that this is a pretty expensive commodity.