> If you make each individual bulletin board receive broadcasts from a central server

Your're doing this with phpBB? Doesn't happen to be open-source somewhere?

Would be interesting to have a look, I think I a bit like this opt-in partial federation / hivemind. Would be even more interesting if it was possible to sync comments between such forums.

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Developing forum software myself, Talkyard. Based in Europe (Sweden).

Started thinking even more about using some European cloud, as an option. There's a Swedish hosting provider that looks interesting (I think)