I think, locality is the key here. Back to the roots of bitchat. But other than "chat only with who's in reach of your antenna", use the antenna to collect and broadcast keys. And then use these keys to limit the chat with encryption! Key leaking will still be a problem sometimes but at least it would work for some time for most locations. My street chat would remain private but some "Pub-key Bar" chat would probably get spammed and would have to rotate keys often.