In terms of the ACPI the g3 state is a mechanical off, but the mac mini has the ability to boot when it senses the power returned. I am trying to put the system in this state after every power down so that when every the mac mini senses a power return it boots, but I am not sure where to look. This is different then g2 state, so please don't bother mentioning anything to do with the system preferences.
Short answer to accomplish what I believe you want to do(?):
Plug the Mac into a power bar with a switch and turn it off (the power bar) when you want the computer to think "the power is out". When you turn the power bar back on the computer will think the building's power has returned.
Warning: Just turning the power bar off without properly shutting the computer down first is a bad idea.
And now a more general answer (as I'm not a cutting-edge computer designer) to the actual question posed, in an attempt to cover all my bases:
The system firmware would be what decides if the system should boot (or not) after PROM power on.
So aside from adjusting the available settings, knobs and buttons in your existing firmware, you'll probably have to look at creating a custom firmware to change it's behaviors.
Warning: Custom Firmware = Not Easy
After reading your other post, the only way you're going to accomplish what you're looking to do is through out-of-band management (The ILO mentioned in the other thread, or something like Intel's vPro/AMT).
There's a reason they have all new technologies to accomplish what you're trying to do (turn a healthy machine that's off to on without triggering an actual power 'failure'), and unfortunately I doubt you'll be able to duplicate it by fiddling with a 'dirty bit'.
I think you're going to either need hardware that supports one of these technologies, or possibly create some kind of third party device to cut power, and return it on command (possibly from remote) so that you can use the Mac's currrent functionality as-is.