I stumbled into a wiki on Turning the Raspberry Pi Into an FM Transmitter.
The code uses the hardware on the raspberry pi that is actually meant to
generate spread-spectrum clock signals on the GPIO pins to output FM
Radio energy. Just add a quarter wave length of wire to GPIO 4 to act as an antenna.
Usage: program wavfile.wav [freq] [sample rate]
Looking at the source, it seems simple enough to narrow it a bit:
In pifm.c look for:
float dval = sample*15.0; // actual transmitted sample. 15 is bandwith (about 75 kHz)
Obviously this isn't a clean transmitter implementation, so don't expect much more than an autofox.
For a low power foxhunt it might amusing to wire a motion sensor to the other GPIO to adjust your transmit and announce timing cycle, when folks are getting close.
Apparently some people think like me:
Plenty of folks using the basis of this for Weak Signal Propagation Reporting