>Use an RS485 interface at the sending end, I think they can supply ~60ma

I suppose I should add that the power I'm thinking of transmitting is
in the 40 volt 20 amp range. Data has to ride on top of this.

I suppose I could use AC instead of DC and have a rectifier/filter
at each node. Then use the X10 method. Any advantage to this?
The length of this system is not so great so the advantage of AC
over DC in terms of the costs of a thicker cable is not an issue.

Michael