If you're willing to look about (I can't find where I last saw it), there's a PSU with a remote control. It can be tuned on and off remotely, of course. The remote itself is a sort of key fob, reminds me of the key fobs from third-party alarm systems in cars.
Not quite sure on the exact specs and such, of the PSU, and how it works, but it seems it would simply connect up to the power switch on the motherboard, or that seems the simplest idea anyhow. Although you have a good PSU already, so there's little point in that.
If you have an alarm system in your home, with a front door sensor, you could adapt that, but you'd still need to do something to make sure the PC is really off, or at least in a low-power state, before it closes the circuit, and lets power flow between the power pins, otherwise every time your front door is opened, your PC will turn off
If you do have an alarm system, you can try and modify the alarm box, to actually send a signal once the door is opened to another receiver. You'd have to construct, or use a separate circuit for this mind, as most alarm systems likely won't be available with receivers, but rather, transmitters, such as the motion sensors.
The receiver on your computer's end could then have logic circuits inside, connected to the power LED pins, and also, subsequently the power switch pins on the motherboard. The circuit could be made to check for power on the LED line. If there's power, that means the LED should be on, so the system is turned on, and no signal is needed.
I there's no power through the LED pins, the PC is off, or in a low-power state, and needs to be turned on, or woken up. It can then close the power switch circuit momentarily (with the logic detecting the LED line getting power, it should then open the power switch circuit again, so that it's not emulating when the power button is held down).
In this way, the door can be opened even when the PC is on, and the power switch won't (or shouldn't at least), be activated again.
I've probably got something completely wrong somewhere along the lines, but that's how I'd do it, without asking for further help. Someone's got a better system out there though I bet.
