Having spent a while talking to @catrope at the 2023 Hackathon - we did some further debugging on Echo.

• Filed T337185 upstream for fetching notifications content from "foreign" wikis. Currently, there's a requirement on CentralAuth that sends a token to the foreign wiki allowing it to access the user's notifications.
  • Some SessionManager consideration
• Look at SiteConfiguration to trick $wgConf (to be filed as a separate task)