Answering my own questions, partially:
The canonical list of fonts available through ULS is: https://git.wikimedia.org/tree/mediawiki%2Fextensions%2FUniversalLanguageSel...
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