Thank you.  I'll see if I can tune the query to do this efficiently.  The cross-db issue comes from joining the Event Logging table with the mediawiki table.  If my tuning doesn't yield results, the only viable solution is to import the event logging stuff into a temp table in labsdb/enwiki_p.  Then they'll be on the same database and the query should fly.  Is that possible with the schema you're capturing for mobile registrations?  In other words, can that data be shared publicly?