Cloudron already serves some well-known content per domain, but there's no way to set the arbitrary JSON that Matrix needs today (e.g. the org.matrix.msc4143.rtc_foci transport entry that Element Call requires), and no way for a package to register it automatically. A wellKnown section in the manifest (updated on configure/restart) would benefit any protocol using apex-domain discovery — Matrix, WebFinger/Mastodon delegation, etc. Happy to write up a concrete proposal or beta-test an implementation with this package.

Is this the same as https://docs.cloudron.io/domains#matrix-server-location?

Almost the same, with two differences:

1. That feature only serves a fixed set of locations (matrix/server, matrix/client, host-meta, caldav/carddav). element/element.json isn't in the whitelist, so adding it to the domain config today would still 404. If you're open to adding it there, or better, serving any configured location, that solves it. I actually have a small patch for the generic version ready (branch wellknown-generic-locations on https://git.due.ren/andreas/cloudron-box, incl. regression test) if you want an MR.

2. The entries are static text the admin maintains by hand, while the correct values (e.g. rtc_foci for Element Call, the MAS auth block) come from the app and change with its config. That's the angle of my feature request: letting an app declare/update these entries for its apex domain itself, so they can't go stale. The dashboard feature would stay as the manual override.

So: same mechanism, just asking for it to be un-whitelisted and optionally app-populated.