Thanks for your ideas, studioj. I Googled a few of your suggestions and finally came up with an old list post asking a question similar to mine. One of the responses involved invoking
aplay -L from the alsa-utils package.
aplay -L lists all PCM devices, which can be used as the device name in any ALSA-aware program with the syntax
plug:<PCM>. Listings for my card included
default,
front,
rear,
center_lfe,
side, and
surround40/41/50/51/71 for multichannel output. There was also
iec958, my s/PDIF encoder. Similarly,
arecord -L lists all capture devices.
[Edit:
null is also an option, for those who wish to send their output to oblivion.]
After plugging my headset into the rear channel jack, both Twinkle and xine could be configured to use the device
plug:rear to push their output through the headset.
I hope this helps anyone else who may be looking to do something similar. It's a lot simpler then mucking around with .asoundrc files or JACK all day.
Cheers,
Brian