Superimpose a data transmission on a custom AC voltage line
You may want to take a look at how modern Fireworks / Pyro controllers handle their power and communications.  One common technique is that the master unit which supplies the power to the other units in the system uses an H-bridge to drive the pair of lines with bi-polar DC.  In essence, the line pair carries a square-wave AC signal that can supply several Amps of current.  Data transfer is encoded in the HI-LO transitions.

Encoding is usually Manchester but could also be a form of MFM (modified FM).

The master also leaves some dead-time between pulses so that remote units can send communications both back to the master unit as well as to other units on the buss.  Obviously, this looks like a broadcast-type communications strategy where every remote unit sees the same transmitted data.  That means that the remote units have some form of addressing so that they can pick off the data intended for only them.