Looking for a simple way to inject a signal on 60Hz Mains line
There are a lot of ways this is done today from high speed power line networking like say a home plug.   To more simple things that grew out of X10 technology for home automation that essentially waits for the zero crossing point and sends some data, to turn a light on and off.   There are many chips and solutions out there you can choose from, just depends on your application and what you like.

Wiki has an overview here: