Mario mentioned that he met someone from Sony, and here I quote, ‘by pure chance’ during a conference (Sust-KDD 2012 in Beijing ?). Apparently he was working on active NILM. The idea, from what I gathered was to send active pulses (or signals) in the power line and look at the response in order to perform NILM. This is in direct contrast to all the “passive” methods so far where already existing signals are studied and analyzed unmodified.
Something of similar sort is done to find faults in fiber optics cables (look for OTDR).
One thing to try would be to look at a simple circuit (with a bulb and a battery). Maybe send an impulse from one point and measure it at some other point when the bulb is OFF and compare to how it changes when the bulb is ON.
Something similar has been done by Patel et al (that guy seems to have done everything) where they send pulses from two (distant) sources down the powerline and use receivers throughout the house for indoor positioning. Although a completely different problem, this can be a good starting point to see what kind of frequency ranges the pulse should be sent at in order to cover the whole house.
My first question is if this can somehow be used to gain distance information (as most devices in the house are mostly stationary and their distance from the main circuit is a characteristic feature in itself). Maybe you could have plug-meters around the house and check for disturbances at nodes. Or even better, maybe you could look at how disturbances in the voltage line due to EMI affect the pulse. That probably would be an easier estimator of distance. Question is, is it worth pursuing ?