Okay, to answer my own question:
https://musicbrainz.org/ws/2/discid/I5l9cCSFccLKFEKS.7wqSZAo...
will give you a list of "releases" for that discid. Use the release ID in this endpoint, with the included parameters:
https://musicbrainz.org/ws/2/release/8e061dc4-790e-4587-ba53...
and you'll get the track data. Not quite sure how to get the album art yet, but should follow.
Cover art, when available, is via the Cover Art Archive API: https://coverartarchive.org/api
(The Release lookup has a `cover-art-archive` JSON object which contains information about what can be fetched from CAA.)
In other words, from a "discid" (in Musicbrainz format or FreeDB format or otherwise), I want artist, album, and track titles, and optimally cover artwork.
Does anyone know how to get that, in JSON or XML format, from Musicbrainz?