During a drag, you are moving a clone of the original activity because we don't want to manipulate the original (just as Outlook does not) and because the original needs to be displayed as well should you press ctrl to indicate a copy should be created. That being said I cannot think of a reason why we cannot copy over the dataitem - at least during the drag operation. Obviously when the drag operation is complete, if a copy is generated it would have a new dataitem resulting from a new item in the source being created. In that case you would either have to use the MetadataPropertyMappings because those properties would be copied over to the new activity when it is created or you would have to handle the ActivitiesDragged event where you will be given the new activities (for a copy operation - for a move you're given the original source activities) and you would need to cache the original activities (from the Dragging event) and copy over what you need.

Note, you may want to consider using the MetadataPropertyMappings and set up your custom AppointmentPresenter to bind to Metadata[yourmetadataproperty] instead of DataItem.YourPropertyName.