It looks like you could use a similar method to the radio stations. They send
 broadcast quality audio over a two wire ballanced line, they have center tap
 the transformers at each end. Then they use earth as one wire and the center
 taps as the second wire.

You could use the same idea.
1) use the phantom power (about 48V) to power the Micros,
2) super impose the data on the power as a pulse per bit time.


I have a rough untested design that uses for the :-
Transmitter == A tristate o/p pin, dc isolation cap, two resistors as voltage
 divider.

Reciver == An opamp, four resistors and a cap.
                 and maybe a 555 ic

Phantom power is often taped off the ballanced line with a couple of resistors.


Regards
Peter Wintulich
Pwintuli@Microbits.com.au