i know i have one of the old 150khz carrier current intercoms in one of my early 60's circuit books. all you would need to add would be an audio tone generator at the transmitter end and a tone decoder at the receiver end to switch a relay (or even better an SSR, since all that requires is a logic level).. it's really not complicated, the transmitter and receiver are just standard AM or FM circuits capacitively coupled to the power line. the only difference is that they operate at 150khz instead of one of the broadcast bands.