DavMail Calendar support is now available, tested with Thunderbird/Lightning and Apple iCal. This should also work with other Caldav clients.
Open iCal Preferences, go to the Accounts tab and click the plus button:
Choose a name, enter you Exchange username and password. Open the server options section to enter the full calendar URL (adjust port to your settings):
http://localhost:1080/principals/users/mail@company.com
Replace mail@company.com with actual user email address.
Click Add, choose Connect Anyway in warning dialog:
Proceed to OSX directory setup to enable attendee completion.