Maybe you could co-opt the kind for NIP-28 channel messages? Although I don't see any harm in making a new kind just for basic MLS messages 

 Oh? Development in private DMs?
Always 👀 for those!

nostr:note1pwrg09w88rld4lqzpq5zy4qped4y8p6ynyrj2pv8ky8gn05ytltqxvfas0