I've built my off panel widget (A custom splash screen) and it's working great in the developer edition. I've also added it to a ArcGIS Portal installation by including the widget in:
Program Files\ArcGIS\Portal\apps\webappviewer\widgets
and
Program Files\ArcGIS\Portal\apps\webappbuilder\widgets
This appears to be working and I can create new applications with the new widget. Existing sites it doesn't appear as configurable in the Portal WAB (until after you modify the JSON of the item manually).
If I add my widget to Portal using the normal method of putting the custom widgets in a shared virtual directory and adding them as an 'app extension' they seem to only behave as on panel widgets.