(I know that Shoutcast, Icecast 1, and Icecast 2 use three different, incompatible, and stupid metadata formats, so I guess I shouldn't be surprised that this is still a clusterfuck.)
Update: Apparently it works just fine in iTunes with the DNA streams for everyone but me, even if I turn off the Mac's firewall. This proves that my "computer malfunction field" is fully operational on MacOS as well. I had been wondering when that was going to kick in.
Update 2: Apparently it's Privoxy's fault! I forgot that proxy settings are global now, and not just a setting in the web browser. Apparently iTunes runs the HTTP proxy for MP3 streams even though they are neither (technically) HTTP nor on port 80, and -- somehow -- that's causing the metadata to vanish.