I am about to deploy a multisite installation of q2a and exploring the possibilities to use different DBs for every site, as well as for the user accounts.
So far I've managed to make two installations sharing single user DB and it played pretty nice:
1. in the config file I set QA_MYSQL_USERS_PREFIX to 'dbname.' (note the dot).
2. in a plugin I override qa_db_query_execute($query) to catch some more tricky queries.
So far, all seems to work fine.
Is anyone interested? I will be glad if someone throw an idea, or help me test it as much as possible.