I'm a little leary of transmitting focused RF power. On the other hand you could put your burrito on top of your phone so it would receive some of that focused power to heat it up.
On a more serious note, I've always wondered if a tiny, very-low-power device could harvest power from all the RF being broadcast all over the place these days: WiFi at 2.4GHz, 5GHz, all the various cell frequencies, even the old UHF,VHF radio and television.
Certainly it is possible to build a tank circuit to capture some of this power. I'm just wondering if it is enough to do anything with. Maybe you could have a sensor that just collects miscellaneous RF power for a few hours, then it could broadcast its data for a few microseconds.