Usage of abstract base class registered in QML

[Yosemite]

I've have a Q_OBJECT derived C++ base class, which is registered with QML using qmlRegisterType. This registered type is a property of a QML item, which defaults to null, like so:

Item {

    id: root
    property QtBaseClass implementation: null

In turn, there are many QML Items that contain this QML item. Each of them sets the propertyimplementation to an instance of a class derived from QtBaseClass.

QtBaseClass itself has no useful function definitions, so I'd like to make it abstract. However, if I make it abstract and register it with qmlRegisterInterface or qmlRegisterUncreatableType, I can't use it as a QML property in this way, even if it is set to null.

What is the intended method to accomplish what I'm doing?