Hey  nostr:npub1s6z7hmmx2vud66f3utxd70qem8cwtggx0jgc7gh8pqwz2k8cltuqrdwk4c! Thanks for taking the time to write this up. I was thinking the exact same thing. I have been in touch with various MLS folks (both those working on the spec but also those working on the OpenMLS implementation). They are open to the idea and suggested that we create it as a custom ciphersuite so we don't have to wait on IANA to give us an identifier. 

For now, I'm more concerned with getting the rest of the NIP finalized and implementing it with the default ciphersuite to make sure that we have a working solution. As you say, we can deal with the custom ciphersuite later and upgrade groups.