A question was posted on how interference will be handled.  It is a good question.  When watching the video I posted it looks like my voice is causing interference with the (almost) ultrasonic tone.  I think what might be happening in the video is the microphone amplifier that I am using in the proof of concept (MAX9814) has automatic gain control which is amplifying my voice and attenuating other frequencies - including my 18k tone.

I believe that moving to more appropriate hardware for these ultrasonic tones will alleviate this problem.

Another problem to worry about is what happens when the tone that the receiver is listening for is being generated by something else in the vicinity.  This could be handled by providing access to a trim pot that would allow users to select a different frequency.  The problem, though, is that the transmitter would also need to be adjusted.  This would get tedious very quickly.

A solution that I am currently leaning towards is having the wireless wires require/emit two different ultrasonic frequencies.  Both need to be detected by a receiver in order to turn on, and when a transmitter is set high both will be generated.