Using a separate passdb per service

Situation: one front-facing server running Dovecot as IMAP/POP3/ ManageSieve proxy, a mixture of IMAP servers (Dovecot, Exchange, ...) in the back-end. Dovecot's passdb does lookups against MySQL which contains a simple user/host mapping, the actual authentication happens on the back-end IMAP servers. The configuration is more or less as described here: http://wiki2.dovecot.org/PasswordDatabase/ExtraFields/Proxy

Now I would like to add a Postfix instance on the front-facing server which listens on the submission port and authenticates users via SASL using the local Dovecot's UNIX socket. The idea being that a user only needs to remember one single hostname, one username and one password for all mail-related services.

The problem is that Dovecot is operating in proxy mode, which means that the password_query returns NULL as the password and explicitly returns a field "nopasswd" containing "Y". Thus, users can not authenticate against the UNIX socket.

What I think I want to do is convince Dovecot to use one passdb for the imap/pop3/managesieve services and different one for the "auth" service.

The configuration snippet below doesn't work, but it should illustrate what I want to achieve:

protocols = imap pop3 sieve

service auth { passdb sql { driver = sql args = /etc/dovecot/mysql-auth-sasl.conf.ext }

unix_listener /var/spool/postfix/private/auth {
    user = postfix
    group = postfix
    mode = 0666
}

}

# IMAP/POP3/ManageSieve auth against MySQL passdb sql { driver = sql args = /etc/dovecot/mysql-auth-default.conf.ext }

Example mysql-auth-sasl.conf.ext

driver = mysql connect = host=127.0.0.1 dbname=mail user=mail password=somethingrandom password_query = SELECT password AS password FROM users WHERE login = '%u'

Example mysql-auth-default.conf.ext:

driver = mysql connect = host=127.0.0.1 dbname=mail user=mail password=somethingrandom password_query = SELECT NULL AS password, 'Y' as nopassword, host, 'Y' AS proxy FROM users WHERE login = '%u'

Any pointers?

Gerry