first, you have to contend with the inverse square law, and most of the large transmission lines have a right-of-way easement that you would not be able to put anything on (if you did, you could get ticketed for trespassing), so there is a limitation on how close you could get your device.
second, the actual radiated field around transmission lines is limited by the fact that the AC field cancels itself. the field around a single wire is canceled by the net field radiated by the other two wires (3 phases, remember).
you might be able to pick up a little useful energy from ground stakes placed a mile or more apart. this wouldn't be from induction, but more likely from leakage paths, and the amount of energy would vary with the weather and with the loading of the system. even an inductive "pickup" would suffer from variations in the loading of the system, since the strength of the field is proportional to the current.