I just realized that aside from simple copies, the ALSA route_policy duplicate will mix to arbitrary numbers of output channels AND that such a device can use a Dmix PCM device as its slave. This means that it's possible to take 2 channel CD audio and have it mixed to 5.1 channel surround, and still let other applications use the sound card. This makes XMMS very happy.
On the other hand, my onboard i810 sound card reverses the surround and center channels, and it does some funky mixing on the center channel for the subwoofer, which sounds really messed up when played on the rear speakers. I haven't figured out how to compensate for this yet.
A useful ALSA FAQ can be found here: http://alsa.opensrc.org/faq/.