We recently set up an ownCloud hosted site as well as a mailserver based on postfixAdmin. In order to authenticate ownCloud to postfixAdmin, one can use either user_imap oder user_pwauth. Both methods work, but with the drawback that a user list cannot be retrieved.
In order to overcome this limit, I developed a simple app for ownCloud to authenticate against an arbitrary mySQL database. Currently, only the crypt() password method is supported, but database, table and column details can be configured in the admin backend.
You can find the details (and the code!) in my mercurial repository at http://www.aboehler.at/hg/user_sql