It would be great if this idea could be made to work. Unfortunately you can't overrule laws of nature with an act of congress. The problems of coupling signals across the transformers at the substations, the fact that more and more high voltage transmission lines are HVDC with solid state switching, the wires themselves are open with no EMI shielding, cross-talk between parallel lines, plus all the other noise and garbage you find on powerlines from all the stuff that Joe Blow has plugged in combine to make this more a dream than a practical plan. All the big demonstration projects I have read about in the technical journals have been more amusing that anything else. You can find a lot of testimonials and anecdotal reports but no test data with measured data rates, or professionally conducted trials.