Not to rain on your parade but....d
I'd be willing to bet it's not switched, that is, it's hot all the time if the main switch on the PS is on.
I searched high and low couple years back when I built my cab for a ATX PS with pass through cable for monitor. I found a couple but both I purchased the extra outlet is hot all the time if the main PS switch is on (even when the computer is not).
I spoke to a couple of techs (and friends) with electrical engineering backgrounds and they indicated this is by design (obviously) based on the design of the ATX PS itself.
I ultimately went down the path of wiring a relay into a powerstrip controlled by a 12v line from the PC. Power on PC, relay triggers, powers on everything else.
Just trying to help and save you some $$.