[Dovecot] Allowing non-SSL connections only for certain Password Databases

I'm trying to set up my system (using Dovecot 2.0.9) so that certain Password Databases are available over pop3 and imap and others are available over pop3s and imaps.

In particular, I would like users to be able to connect without SSL to accounts set up in vpopmail, but to require SSL for system accounts.

Is there a way to set "disable_plaintext_auth" to different values for different Password Databases? Is there another way to do it?

Thank you, Dan