According to Jens, the MediaExport plugin does not all adding of child containers vi UPnP's CreateObject action, though it (and the tracker plugin) does allow adding of child items.

When implementing this, we would have to change (or add and deprecate) WritableContainer::add_item(MediaItem) to add_object(MediaObject) and add a runtime check to the tracker plugin, which has no concept of adding to child containers.