Quick note to self: remember to check the issue queue(s) when you've got an issue!
A little time ago, Picard (the official MusicBrainz tagger) decided to stop being able to look up CDs (and thus stopped being able to add DiscIDs to the database). After all the usual debugging, I asked if anyone in #musicbrainz could help me onwards, but alas, they were only able to suggest things I had already tried. This was yesterday.
Today I went to Gentoo's bugzilla and looked at reports related to Picard, and lo and behold if I did not happen upon a bug about how the Picard ebuild doesn't contain a proper dependency for libdiscid, or, more to the point, that the Picard 0.10.0RC1 ebuild wants ">=media-libs/libdiscid-0.1.1" with the latest available is 0.2.1 and that Picard is looking for "libdiscid.so.0" while libdiscid-0.2.1 installs "libdiscid.so.1" and no "libdiscid.so.0". So the solution was simply to go back to version 0.1.1 of libdiscid, and Picard is now looking up CDs as well as always. :)
(I'd also still like feedback on what to do about Farore's OS X.)