I am running DavMail on a server and I synch my work account through it to various clients.
Google calendar does not honour the login request via caldav to DavMail, so I can't currently sync OWA to my gafyd calendar using DavMail.
Could you hold the exchange credentials at the DavMail server and respond on behalf of a client that does not perform a login by itself?
It would mean you could use GAFYD instead of Outlook as the front end for an exchange system which would make a lot of people happy!
BTW If I can help with this or help test it I am happy to give it a go.