There's no BMS player available yet, that I've heard of. However, I do know that the format is being looked into. Currently, the AW files (which are sound banks) are being worked on. Once the format for those is figured out, I think the BMS files will be next.
One thing I think I should mention is that HCS has been busy lately, so he's not the one looking into the AW or BMS files. When he gets more time, he may do so, but not right now. In other words, the person who's looking into AW and BMS files isn't as experienced as HCS. As such, don't hold your breath for that at this time. If you're really interested though, feel free to join us at #usf on irc.freenode.net (I'm using mIRC), or the HCS forums
here. And yes, the ability to play back the sequenced GC music would be far beyond awesome... Mouser X over and out.