I'm not sure what you are trying to obtain but if you just want to mark a widget that has a property set, apply an event filter on your application object and whenever you receive a paint event, draw the widget as usual but also draw a border around the widget like Designer does with layouts,
How to draw the border ? We will receive the paint event in eventFilter, then where to draw the border ? we cant draw outside widgets. Can you show a psuedo code ?