Reading old topics again it seems like this wouldn’t need a plugin at all. For reference, here are two posts that mention the idea:
This approach could work for categories like the “Lounge” where access is based on trust level – you could set anonymous posting min trust level to the same as for viewing the category. It would mean, though, that fewer people could use anonymous mode in the first place.
But I can’t figure out a way to allow anonymous users to access private groups in any other circumstance
