Simple Power Line Communication
If the old system works on either 12VDC or 24VDC, how about using 12VDC then wire a 24VDC relay at the opener circuit.  Then when you want to send the open signal you would temporarily increase the line voltage to 24VDC, (at 24VDC the relay closes).  You would have the relay contacts switch the same line voltage over to the opener circuit.  If the relay partially closes or remains closed at 12VDC you might add a high power Zener diode near the relay coil to improve the operation.   

As an alternate to this a simple voltage detector could be designed.  A stable reference voltage could be generated and the high or low DC line voltage compared to this. As with the relay idea, a simple high voltage detection signals the opener to operate.