Sorry, I didn't fully understand on the first read. This is actually interesting, maybe NIP 66 would help? Trust is an issue though, seems weird to trust a relay based purely on geographical proximity. You would also have to probe them to know they would accept events.

What's brute force about it? I don't want to find specific users, I want to talk to a location-based channel.

Yes for gathering the first step where we gather a relay list it seems like a good option. Do you have a script that can crawl for relay lists?

I've written a few but they are all terrible. I do think maybe self-attested nip 66 would be better here. That way you're not hijacking random personal relays, instead relying on volunteers who want to serve a particular geography. You would only need the usual 2-5 per region, and a few geo "hubs" could do a lot of the lifting.