Just a thought...
Power lines sits at 12v normally, PIC and associated circuitry runs off 5v or less, so maybe it's got a regulator running to it, or it's running off a battery pack that's slow charged off the normally 12v line...whatever might work.
Modulate (send the data) the power line between 12v and 5v (i.e. 12v = 1, 5v = 0), use an op-amp at the RX end to 'subtract' 7v from the power line and feed that output into the RX pin of the PIC.
Got no idea how I'd do it off the top of my head. Could probably come up with something if I'd put my mind to it.
But it's just an idea that might get you going.