One of the first things I did when I heard of nostr was experiment with a protobuf mapping to reduce bandwidth. nostr:npub1xtscya34g58tk0z605fvr788k263gsu6cy9x0mhnm87echrgufzsevkk5s later gave me the helpful hint that strfry already maps to flat buffers internally (afair). So that may be the easiest way to go. Just modifying a strfry to serve flat buffers.