[2.0],[3.0] Using the OnPaint for a Button

What i'm woundering is can you use the OnPaint to Draw say a 1/2 Circle that is white,set the Opacity to 30% lets say.But OverLay the Button with that 1/2 Circle using "Bring Forward" something like that..

I know if it can be done it would have to be done programatically,so can someone give me a starting point for this??? :)