What is the current story for serving jj repositories over HTTPS? Git has https://git-scm.com/book/ms/v2/Git-on-the-Server-The-Protoco... . Does jj have something better, equivalent, or nothing (at this time) at all?

Unless you're at Google (or writing your own backend), jj talks to a git server, and so you'd just do exactly that: host a git repo over https.