I had a little play with dropping a Path (made from some text in Expression Design) over the top of a video and then using mouse drag in order to skew the Path left, right, up and down.

It looks like this;

and it's here: http://mtaulty.com/wpfe/ex8.html

and you can try and mouse down and then drag the mouse left, right and up,down to change the skew (the details of which should print at the bottom of the screen).

It's very basic but I'm drawn to this idea of putting things "on top" of video content.

Update. I showed MikeO this and he asked "what is that thing then, a piece of text?". Essentially, all that's on the screen here is;

Canvas

  MediaElement

  Path (the Path is the black rectangle with the word "Video" cut out of it)

  TextBlocks at the bottom

The way that I made the Path was to use Expression Design and do;

1. Insert a piece of text with the word Video on it (set the font, size, etc)
2. Convert the text to a Path using the Object menu
3. Draw a rectangle around that newly created Path
4. Select both the rectangle and the text Path and make them into a Compound object (using the Object menu)
5. Fill the newly constructed compound object with a black brush