Adding sizeChanged and NOTIFY for QGraphicsWidget would enable property binding in QML. This would allow the following to work;

Image {
  source: "background.png"
  width: label.size.width // binding to graphics widget's size
  height: label.size.height
  MyLib.Label {
      id: label
      text: "Hello world!"
  }
}

where 'MyLib' is a set of components based on QGraphicsWidget. However, it should be pointed out that heavy usage should be avoided as binding mechanism is not efficient compared to direct resize calls used by qml anchors and graphics layouts.