My first instinct is that the signal probably doesn't need to be ultrasound. In the same way that my phone will sometimes hear my car radio and falsely hear, "hello Google", I bet the beacon can just be planted on top of the signal and match filtered to a decent SNR. Similar to image steganography, it doesn't need to be perceptible to the listener. Compression issues would exist, but at least they wouldn't be fighting against the transmission hardware.