I have tried to use extended properties on appointments with EWS, but I can not seem to find the appointments again. The set property part is equal to the one shown in this
here's a samplecode how to create an appointment with the customid and find it after saving:
ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2007_SP1);
service.AutodiscoverUrl("someone@somewhere.com");
ExtendedPropertyDefinition def = new ExtendedPropertyDefinition(DefaultExtendedPropertySet.PublicStrings, "AppointmenId", MapiPropertyType.String);
Guid testid = Guid.NewGuid ();
Appointment appointment = new Appointment(service);
appointment.Subject = "Test";
appointment.Start = DateTime.Now.AddHours(1);
appointment.End = DateTime.Now.AddHours(2);
appointment.SetExtendedProperty(def, testid.ToString());
appointment.Save(WellKnownFolderName.Calendar);
SearchFilter filter = new SearchFilter.IsEqualTo(def, testid.ToString());
FindItemsResults- fir = service.FindItems(WellKnownFolderName.Calendar, filter, new ItemView(10));
hope this helps you...