How about having URLs that begin with njump: or nostr: Then we could have apps and browser plugins that handle it according to the user's preferences

On Android, I think it's possible to make PWAs that can respond to these custom URLs and which will work offline

Yeah, that way we wouldn't need to depend of the "njump" used by the client, but, as a Keychat user, this would be hard to me (no browser plugins on @Keychat)