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Travis did set me up with an Exchange account for testing, and it turns out that the Sync Services API reports that the Exchange calendars are read-only. Unfortunately, the Sync Services API is the only calendar API which is compatible with 10.4, so until we drop 10.4 support (in OmniFocus 2) we can't sync with those Exchange calendars (unless Apple adds support for Sync Services writing them, not just reading them).

Sorry for the bad news!