Automatic power on when AC is switched on?

How can I configure the iMac firmware to power on automatically when the AC is switched from off to on?

(I've asked a similar question in the Mac mini threads, but here the situation is different. The power button on the iMac occasionally jams in. I plan to get this repaired, but I'm in a remote area and it may be a long time before I can schedule the repair. When the button jams in, the fan in the iMac may race and the display shows nothing; presumably a firmware test mode. I have to give the button a tap or two so that it pops out. It is hard to tell if it is jammed in. So I'm looking for a way to workaround the problem by avoiding the button altogether. It would also make my daily workflow easier.)

I've asked Apple technical support, but the technician was only aware of the +System Preferences+ -> +Energy Saver+ -> +Start up automatically after a power failure+ checkbox, but I don't want to use this because it would only work if I turned off the AC power without using +Shut Down+ first. The technician agreed that this might be bad for my data.