I'm afraid I don't think there's an easy way to do this.
Probably the best solution would be to write your own login/register/user functionality, and then use Q2A in the mode which integrates with an external user management system. That way you could centralize the logins. There's more information about that on the Q2A 'Advanced' page.
Another solution could be to centralize the qa_users and qa_userprofile tables in one database, and then use MySQL's FEDERATED table type to allow the other databases to access those central tables. I'm not sure that would work with everything Q2A does, but it seems quite possible.