Energy harvesting from AC wire
It's certainly possible - one of the energy monitoring sensor companies one of our clients uses has clip-on smart meter sensors which harvest energy from the circuit they monitor to power sensors. There's a minimum current that they can sense, below which they don't harvest enough power to talk to the base station.

I don't know off hand what a standard clamp-on transformer will provide in terms of power, but ultimately their sensors can't have anything much different in terms of a transformer ( possibly more turns, but then size limits the possibilities ). Get a conventional clip-on transformer and measure what you can get out of it.