Hi, if your game over occurs from the map, you can use event commands to play your webm movie before calling the game over scene. If it occurs from battle, I think you could also achieve this with my [CGMZ] Battle Result Events to cause the battle loss to run a common event which could then play your movie and call the game over scene when done.
If you mean instead of an image but a movie as the game over screen background, that is something I would need to look into, I have not done much with movies in rpg maker.