Rockbox looks like it might use one hobby project of mine.
There are not a whole lot of open source headphone filters, but I integrated one of mine into pulseaudio last weekend. It's nothing very fancy, just a simple trick that limits the extreme separation of the stereo image by mixing attenuated, delayed and processed copy of the audio from the other ear to the other ear:
I guess it's fairly similar in idea to what bs2b sounds like, but very, very much simpler, something you could easily turn into an integer version and add somewhere in the audio stack. I integrated it into pulseaudio, myself, and have been using it for a few days.
Numerous 15 second clips are provided on the page for evaluation purposes.