Interesting. So I would ordinarily want to put a foreign key constraint on the user_id column of a UserSessions table (or similar). In general, presumably you have relationships across the tables that are in those discrete databases. Do you just enforce these constraints/do joins in code? It seems like splitting related tables across multiple databases loses some (possibly a lot?) of the benefits of relational DBs, so I'm curious how you handle/manage that.
That said, I love the idea this architecture. Might use it for whatever next dumb little web service I cook up! I love how this simplifies a lot of dev/deployment ops, perfect for a side project.