Eric,
The ability to use related data fields is not part of this widget as you know. Adding related fields would be a lot of code changes to the widget, as right now the fields that are listed come from the FeatureLayer and then the chosen field (configured field) is used in the layer.queryFeatures which would have to change to a relationshipQuery based on it be a related field (which means logic has to be added to determine which fields are coming from a relate). Bottom line it is could be done but would require a lot of work.