Data over DC Power, mono-directional, simplest circuit
I suggest looking for ready-made DC-BUS or DC-LIN module instead of making something yourself.

For example Yamar Electronics specializes in DC-BUS communication and their SIG60 chip is available on ready-made modules like this one.

However in case you want to spend time and money on it, this application note has everything you need.

Having said that, why would one invent a bicycle instead of using ubiquitous DMX hardware is beyond my understanding...  

Keep in mind that all PLC solutions create quite a lot of EMI, especially with lines spread all over large area as in this case. There is a chance they will interfere with other equipment, like wireless mikes. Then you'll be responsible for failed performance. There is a reason DMX cables are shielded.