What you really want is the flash video player to make an ExternalInterface call to javascript letting it know it has completed. You would need access to the flash file to accomplish this.
You could technically track the video play progress with javascript and load the next video on complet. The problem with this is that it would require you not only to know the length of the video, but also when it plays/stop. The only way to do that is by having flash make ExternalInterface calls. At that point, you might as well just listen for the video's complete event.