CouchDB 1.1.0 and upcoming 1.1.1 do not support this. The best you can do is create a public database in which users send requests for user accounts, and you have a separate tool for responding to this and creating regular user accounts. (A stand-alone tool may simply be a specialized webpage that you log in as an administrator.)
I introduced a patch to do what you described. The CouchDB incoming message database is called , it allows unprivileged users to write some approved documents to the database, but they cannot read any data. Creating new user accounts is why I made the fix.
, 1.2, 1.3.