Yeah I have thought about that - but figured that it wouldn't be much different to what's happening now with my current transformer. With a 100W lamp on at 100% the current through a multimeter is 390mA - curiously as the % drops the current flow rises to a peak of 433mA - this is due to the combined lamp load and the capacitance of the cable when faced with the sharp turnon of the triac part way through the phase cycle. When the lamp has dimmed down to 3W (filament extinguished) there is 135mA of current flowing (this in my mind is the capacitance of the lead) - in perfect world would be 13mA. When the light switch is off and the dimming phase is part way, multimeter reads upto 180mA - which makes it impossible to figure if the light switch is turned on or off by current alone.

This is why i was figuring I should send some sort of 'tone' down the line and listen for it on the other side - whether it be at zero cross only or all the time, I'm just not quite sure how to do it having such a large potential either side of my circuit.