I would strongly advise you against modifying the 4 corners of the widget. The 4 corners define the widget's area, and are meant to be a rectangle (which is why UIWidget derives from UIRect!).
If you want an irregular widget, that's fine -- it's up to you what's drawn after all. Have your custom widget define other (internal) points that reside inside the rectangle which you will then use to draw the widget in some fashion.
I recommend keeping them in 0 to 1 range as well, 0 being the bottom/left, and 1 being top/right. This way it will work properly when the widget's dimensions get resized.