Really lame idea:

Use an RS485 interface at the sending end, I think they can supply ~60ma
to the load.  Interface is differential, you have 2 wires A & B lets say.
Take A & B run them through a rectifier and big capacitor, and you have
relatively smooth power (might need to regulate).  Then use an RS485
receiver on the receiving end to get async serial signals at whatever
rate.  Best of all it would seem nearly impossible to violate the
common-mode grounding specs since the signal lines are your ground.

This is kind of unidirectional though unless someone has a bright idea
about that.