e.g. magnolia.publishing.receivers, in order to add/remove public instances at runtime.

Interestingly, that's on purpose with the default source for external locations - see similar thread on quarkus (using smallrye config internally as well).
—even if config was read on the fly, some Magnolia components would still need to do something about it (we had some in SaaS too), except in quarkus, restarting is less of an issue.

Instead, we may need a custom ConfigSource then, there are a few pluggable options as well for those who need to work around it.

Then on application side, verify few things/options:

• does publishing keep receivers state somewhere else? (do we need hooks to account for updates)
• can the actually injected config be bytebuddy proxies, to reload arbitrary config on the fly

Workaround

Use system-properties for values that require updates + set config_ordinal property lower than 400 in your MP config file (by default inherited from how you configure smallrye.config.locations, typically via sysprop = 400). More info here.