There is a proposal by nostr:nprofile1qqsf03c2gsmx5ef4c9zmxvlew04gdh7u94afnknp33qvv3c94kvwxgsppemhxue69uhkummn9ekx7mp0qyvhwumn8ghj76rzwghxxmmjv93kcefwwdhkx6tpdshsz9thwden5te0wfjkccte9ejxzmt4wvhxjme0sa...
Sounds like you need a NIP to specify how to privately send a tree of that that identifying metadata so a server can know what to securely send. That might help reduce the size NIP-17 traffic.