Ideally idiomatic nostr, it's times like this more crypto knowledge would be useful. At least it's easy to bandy about terms like Schnorr and accumulator. Communities or like you said a server may be the way to do it as v1. nostr:npub1l2vyh47mk2p0qlsku7hg0vn29faehy9hy34ygaclpn66ukqp3afqutajft says nSecBunker.com could be used.

The idea I'm thinking of is a bespoke community based on place. I post a QR code somewhere and then people can join a community around Trafalgar Square or Central Park and anyone could post in it and your normal moderation tooling (WoT?) or blocks would apply. Like a physical town square. The thing that would be nice however is a revocable secret required to join so only people who'd physically been there and scanned a current QR code could join.