The 'defaultExceptionWorkgroup' field on the DocumentType BO is set and used only during XML parsing. There is no OJB mapping for this field. For DocumentType XML export, this field is reverse-derived by the following heuristic: if all nodes have the same exception workgroup as the first node, then this workgroup is taken as the default exception workgroup.
This is more in the "cleanup" category. If this was fixed so that the default exception workgroup was actually stored (and then directly exported), would we still need to keep the heuristic for existing documenttype definitions, or could those be fixed up somehow? Is this worth solving, or is it simply a matter of documenting (or removing the BO field)?