Of the order of maybe 20W average, 100W peak. It's an LED lighting array. And either 5 or 12/24 volt
This might be a challenge if you want to do RF over power line. You'll need a choke that can handle 8-9Amps and still operate as an effective choke at 400+Mhz... I'd say trying to do that AND match for 50ohms is going to require some engineering. The good news is that the radios have a pretty large dynamic range and might be forgiving enough to handle this application.
The idea of using 2 wire is simply to reduce complexity and cost. 3 wire is feasible, and I'd just use RS232.
This seems the least complex...
One Wire Protocol is another option, if you need bidirectional signalling. There might be some One Wire GPIO expanders. I looked for a one wire expander and didn't find anything that seemed more useful than RS232. If you need bidirectional over a single wire, it would be relatively simple to set up a half duplex protocol that would work.
I did look at baseband modulation of the power line (low baud rate, signalling by polarity swap), but that would involve big mosfets in the driver.
Yeah, that seems like a lot of extra HW to do what you want to do. However, if you could turn OFF the load while signalling, then a relatively small MOSFET could be used at both ends to isolate the load from the signal wire and you wouldn't need humongous drivers or a complex network for the signalling. A small battery or supercap could keep your Mote alive during signalling. In this case you wouldn't even need a radio and you could get away with 2 wires. It sounds like you could easily complete the transfer in just a few milliseconds so you may not even notice that the LEDs are powered off...
Tom