I'm doing something similar in one database. I had to ignore couch's built-in user system and roll my own. Couch's built-in permission control is too limited. I found it simple to create a doc type of 'user' and save the doc id in cookies. Then I keep a list of permissions to other docs in my user doc. Then on every access I load the user doc and the use the permissions to control the db lookups (view keys). This is no more complex than what I used to do in mysql.
I know other people have used one db per user, but I found that too complex. How would you do replication?
