I can't remember how fountain does theirs, but in general there's a podcast 2.0 attempt to get cross app comments off the ground, which predates nostr. nostr:npub1qx6kgzfm3z5qlffxnunsgnsaagwcxt6klx6q7gry56arj5r0yl3qgkqf4d would know how feasible your request is.