What you are looking was very common practice in power distribution networks upto and for some time after radio communications, SCADA and other systems started to come into play in the distribution control, and communication with generating sites.
The difference being that you are looking for the low voltage side of things where as the old systems were on the 8KV and up type of systems. And it was very clever with signal routing being determined by frequency and at tie points they had special electrical networks that would shunt the signals to specific branches. I know some of these systems were still in operation in North America well into the '80's (but certainly not mainline systems)
I mention this because this might be the best place for you to look for either old eqt. or even to find papers and techniques of doing this.
Most modern solutions will have many more layers and levels of sophistication than what you need. So looking to these earlier sources of information will help you the most if you want to build something similar.
That fact that these were robust systems also should be reassurance that you can make your own if you are sufficiently experienced /careful when dealing with mains voltages. A modulation scheme and absence/presence detection with robust filtering should work.
There were attempts a setting signalling standards for power-lines, and you might find some of the older modules for transmitting and receiving (even if you are doing just carrier detect) which was made by Echelon systems. They still exist and sell modules. But this might be a too pricey solution.
On-semi and Cypress both sell Cenelec Band-A and Band-B module solutions as semi-conductors. So the app notes there will certainly be useful. However, these solutions may very well be way too much. But you might be able to find demo kits at low cost.
TI has an integrated PLC front end PLC = Powerline Communication which you could use by itself without using all the protocol stack.