I have not used the built in video capability, but from the top of my head:
How about using a library like http://www.videojs.com/ .
The videojs-library can fire an event when the video is done playing ( Like displaying the login form ) :)
Have not tried this myself but if it works, you could even add this functionality to the Internal Captive Portal by using this library :)
Edit:
Just tested this and it works!
Of course this library depends on the html5 <video> tag so your compatibility list would probably be something like:
Internet Explorer 9+, Firefox, Opera, Chrome, and Safari.
You can of course create a fallback so that the users with non-compatible browsers will get to the login form anyways