Hmm. It may be worse than I thought.
Users need to create an account on wordpress.com to set up or comment on a wordpress.com blog. So users would create an account on the Food forum that they could use in the Burger, Pizza, and Hot Pocket forums.
Hmm. Good point about digests. Maybe the “main” site would have no (or few) categories of its own to solve that problem. The food forum would have only announcement categories that affected all communities.
It is a worse hack than I’d at first envisioned, but I think having a way to build groups of communities could be useful. Periodically someone posts about problems with huge numbers of categories and how to deal with them.
Something like this could be a solution for those as well. Big Company might have separate Discourse instances for each Department, but a single user database so that people could cross over if they needed (of course Big Company would have their own SSO). It would be a hack around the only-two-levels-of-categories limitation (which almost always makes sense).
Hack, yes, but maybe not terrible.