Designer plugin - creating a container widget

[yop]

Hi everybody,
has anyone created a container widget in a designer plugin? I mean creating a plugin that when overiding the QWidgetPlugin::isContainer() returns true? I have created a widget that just inherits QDialog and nothing else, the plugin gets integrated nicely but the resulting widget is not editable from the designer. All the custom widgets, that are not container widgets, I have plugged into designer are working nicely.