[Dovecot] proxy: can I use the password returned from passdb to log in to the back-end?

I am hoping to support encrypted passwords, which I know is generally not allowed in a proxy setup. However, I can return the password in clear-text out of the password database, so I was hoping for something similar to the destuser field.

I have successfully used the master password functionality to support encrypted passwords (users impersonating themselves), as long as the back-end server supports that. However, I need to proxy POP3 to Gmail, which does not.