[Cortig]

That would require hacking into WebDAV to get to the accounts themselves and then read-write capability for the calendar.
It sounds like something pretty tedious to implement.
Entourage 2004 can sync to address book and iCal. I suspect it would be far easier to have OmniFocus integrate with this resources and let Entourage do the dirty job of synching it through SyncService into its own database and then onto Exchange.