I have been using doc4j for both html->docx4j and for docx4j-> html conversions.
It will be helpful if you include the following change in you next release or else we need to maintain modifications separately.
CHANGE 1
DocumentPart.java : setPartShortcut : Please add the below else if
public boolean setPartShortcut(Part part, String relationshipType) {
		
		}else if (relationshipType.equals(Namespaces.SETTINGS)) {
				documentSettingsPart = (DocumentSettingsPart)part;
			return true;
		} else 
                {
			return false;
		}
	}
regards,
krithi
			
		
