I don't have an option to try it out, but my guess is this:
Can't you draw a canvas over your video window? Maybe if you set its background colour to fully transparent you won't see the canvas itself, but when you draw something on it, then it will be visible.
To locate canvas exactly over the video window, maybe you can use an another canvas. To be honest this would be working in WPF ( probably
), but if I'm correct in C# 2.0 also should be a canvas class.