I just recently finished this script that turns the raspberry pi into an easy to use and configure fox hunt transmitter. Please have a look at it and give me feedback. It has been tested to work on a raspberry pi model b and it uses nbfm to transmit through one of pi's gpio outputs. It should work on the b+ and I imagine even the model a. I chose gpio pins that are the same on all models. https://github.com/km4efp/pifox
Please see above link for more information.
Any feedback or improvements or bug reports are welcome. Future plans are to combine it with an usb sdr dvb dongle to control it via dtmf codes. Still waiting on hardware to start that project. Let me know how it works for you. A low pass filter is required but that is about it. Optionally if not interested in building a lpf and actually by default you can use the audio out from the pi and vox on a handheld radio as a fox transmitter. Lots of options and configurations are available. Please see the readme section 5 under installation configuring pifox for different setup ideas.