OK, I think I know what you mean now. I don't think what you want is possible without subclassing QGraphicsItem because of the way the Graphics View Framework handles mouse events
All mouse events are delivered to the current mouse grabber item. An item becomes the scene's mouse grabber if it accepts mouse events (see QGraphicsItem::acceptedMouseButtons()) and it receives a mouse press. It stays the mouse grabber until it receives a mouse release when no other mouse buttons are pressed.