There's an addon for wow I used a lot when I played. It's called WoWVid. <--- link to the curse page.
It throws a "black" window on your screen which is making the UI/GUI transparent or something so when you place a video player(VLCplayer) on your desktop you can see what's playing in the adjustable size addon window in game. (I put mine over chat usually >.<'' would probably place it over the datachron now lol.)
I'm not sure how to go about writing code into addons, but it has to be on top of EVERYTHING in the UI and GUI. Adjustable window size (preferably click-corner>Drag)