Required SSL with exceptions

I have a Dovecot cluster which is on separate machines from my webmail/caldav/cardav cluster, and I currently have the system setup with ssl = required. Unfortunately the caldav/cardav server I am running doesn't support STARTTLS so I was wondering if there is a way to still enforce ssl for every connection with the exception of a certain subnet, or if there is a better way to accomplish this without install a local install of Dovecot on each of my caldav/cardav servers.