This is a work in progress
On the teensy 3.1, this implements a software defined radio transmitter that encodes data as PSK31 audio in the 1 KHz range, modulates it using SSB and then uses the builtin DAC hardware to transmit it via a wire antenna.
Due to limitations of the DAC and the current software design, the output frequency is quite low. Roughly 64 KHz, using 8 steps in the carrier wave. This has successfully transmitted a short distance to a SSB receiver and the PSK31 signal decoded by fldigi.
The BeagleBone Black PRU is similar to the teensy design. The userspace SDR code performs the encoding and modulation, and writes the values to a shared buffer. The PRU just does parallel output to an external DAC.
This could potentially use the LCD hardware to clock out parallel data at a high rate, although the extra capacitance from the HDMI framer makes it unsuitable for high-speed digital IO.