signalling/communicating through the zero cross of the AC is trivial as long as there is a galvanic connection (wire). Once you get to transformers nothing much

signalling/communicating through the zero cross of the AC is trivial as long as there is a galvanic connection (wire). Once you get to transformers nothing much happens unless their is a module on the secondary to intercept it and then use other channels to ferry the data. In other words each transformer would need to have a communication device/router - I suppose that's way more expensive than ad-hoc modules per meter.