UNMO3 (available on the MO3 page) can be used to decode MO3 files, and then the output can be played by any MOD player.
There's some example source-code on the MO3 page, using UNMO3 to add MO3 playback support to BASSMOD. The same could be applied to Xmms, or any MOD player.
The next UNMO3 release will also have support for stdin/stdout, so it'll be possible to use without producing any intermediate files.