According to a friend of mine, the Rockbox people are looking into porting it to the Rio Karma.

If there's anyone here who thinks they can help with this effort (they're particularly looking for anyone who knows anything about the hardware platform, and isn't affected/bothered by NDA), please get in touch with them.

You can get them on IRC (#rockbox on freenode), or at http://forums.rockbox.org/index.php?topic=14226.0