I don't think I'm following. There are three possible states: off, standby, and on. You want a signal in which states? I'd have guessed on and standby, thus your dilemma, but your subject line seems to be pointing in the wrong direction for that.

Assuming I'm right anyway, isn't there something you could put in the way of the power cable to detect when juice is flowing? Seems like a relay put in series with that line switching something else would work. But I could be wrong.