Overlay Webplayer With Video Ad

Hello, I know this has been asked time and time again. Simply because I have searched for it time and time again. I began to think it was a little hopeless until now.

How would I go about overlaying the unity webplayer with a video ad from an advertising network?? Every post I had been to in weeks of searching turned up that it was impossible. I stumbled upon this website though: http://www.projuegos.com/juegos/4x4offroad/racing.php#.UAUaMrSe6f4

and the game in particular. It is overlayed with a video ad. Any tips suggestions and especially answers would be of great help!