[Dovecot] External auth database process

Hi, Is it possible to call a completely external process to authenticate users in dovecot?

I am trying to setup a transparent pop3 proxy that will lookup users from a couple of different pop3 servers, authenticate to 1 of them, take a copy of the password and a timestamp and client IP upon success, then proxy to the appropriate end server.

I was hoping I would be able to do something along the lines of:

dovecot.conf:

passdb external
	args = /usr/scripts/dovecot_auth.pl %u %w

The perl script would then carry out the various tasks and return the host and destuser for proxying.

Thanks in advance, Aaron