Send digital data over same wire as analog data
Your idea of using an ultrasonic signal to the audio channel seems doable. Basically you can implement something like RTTY (http://en.wikipedia.org/wiki/Radioteletype) or one of those old 300 bps modem using FSK modulation (Bell 103A or V.21 standards -- see http://en.wikipedia.org/wiki/Bell_103_modem), just using very high frequencies. Modulating the signal with an Arduino might be doable (e.g., generating square waves and filtering them to something closer to a sine wave), but the demodulation part might be trickier (possibly filtering the audio signal to only include your ultrasound FSK tones, and then detecting and differentiating them: you will need a pretty high sampling rate on your ADC).