I wrote a patch to Postfix which now supports SMTP AUTH using dovecot-auth. This time it doesn't need any Dovecot libraries and it's backwards compatible with Cyrus SASL by default. You'll just need to set one variable in Postfix config file to enable it.

Patches and instructions in:

http://dovecot.org/patches/postfix/

Not very well tested yet though. And it doesn't work with 1.0-test59, you'll need to have at least today's nightly snapshot.