Find somewhere you can download iTunes 6 and overwrite iTunes 7?
Dunno if that'll work, maybe unless some behind the scenes stuff has changed as well that affects it.
Try versiontracker.com or macupdate.com for an iTunes 6 download.
Actually, I've already discovered that MacPorts includes mt-daapd, which apparently does exactly what I want. Huzzah!
Now I just need to figure out how to get a program started at boot, and maybe have some kind of watchdog make sure to restart it if it dies.