Answering my own questions, partially:
The canonical list of fonts available through ULS is:
https://git.wikimedia.org/tree/mediawiki%2Fextensions%2FUniversalLanguageSe…
Each directory has a font.ini file which specifies the canonical source of
the font, as well as a reference to a bugzilla or gerrit issue where the
font was specifically requested. From this I can usually manually
reconstruct what language the font is for, and whether it is a "some users
think this look nice" font, "we needed this for the wikimania 2014 wiki"
or
the actual default font for a particular language.
This will be a good resource, but not a panacea. I will still need to
evaluate fonts on a case-by-case basis for each language, and at times I
will probably have to chose a non-ULS font for technical reasons.
--scott
​