I hope I am getting this right – are you suggesting that you are trying to insert a thumbnail to a video with no content in it (0:00 length)? I do not know how this would work. I would suggest you use create a image thumbnail for this post with a video play button in the middle to suggest that this post is a video post. You would set this as the featured image like like you would for any other post.
If I am not getting this right, pls reply as a private message here with the site URL and a temporary login to the WordPress dashboard and I can take a look.