Colin, they are called Ground Fault Interrupters (GFIs) over here. Seems when I looked inside one, both the Line and Neutral wires went through the center of a toroid. What would you do? Rewire it so only one of the load carrying wires goes through the toroid?
That makes sense. The question I have is: can the output of the circuit that senses the torroid field then drives the latching relay in the GFI (which normally interrupts the HOT side when tripped) be used for the OP's purpose? I think it would probably require a little conditioning to be useful. I'd like to know what that signal is, myself.
As I see it, the biggest problem is getting the signals from all these outlets to the OP's computer. Maybe through carrier current transmitter?