jollywhoppers.com
xan.lol
https://witchsky.app/profile/did:plc:q7suwaz53ztc4mbiqyygbn43/post/3mdrzebejzs2j
other apps (and mods) support loading plugins for react native. I'm sure this could be implemented, maybe with a preset trusted library.
Things that could be explicitly extended without 'mods': share sheet options, themes/color schemes, ...