Well, esd turned out to be easier than expected to install, but getting it right wasn't as straightforward.
One would think that using "-port" implies "-tcp" when running esd on the iPaq, but one would be wrong. Worse, one gets fooled because there's no error message when the client app fails to connect: it just directs output to the client's sound system, instead. I thought maybe I'd found a bug.
Once I'd sorted that out, I found the sound quality mediocre on my my little portable speakers, but decided to try hooking the iPaq up to the stereo, anyway.
The results were just awful: I'd switched to a WiFi card, and the music was horribly broken up because there wasn't enough bandwidth to keep up with the 160K bytes/sec XMMS was sending down the non-wire.
Works great via Ethernet, though. Close enough to "CD quality" for my middle-aged ears.
But the WiFi results are a little disheartening, because the iPaq was less than 10 feet from the router, with no intervening walls. I may have to rethink this project, and fall back to old-fashioned analog RF.
Here are a couple of links that turned up while looking into XDMCP:
You might also want to check out fbVNC: there's an iPaq port on Sourceforge.