Your question though, you want to show the first few minutes of the video or something along those lines, then ask users to register?
That'll definitely increase registration. I don't agree with it though. You'll inflate your registration numbers, and at the same time, turn away users from enjoying your content. For example, Quora displays a portion of comments, then blurs the rest asking you to sign-up. I know you can get creative and modify the URL to get the full page, but I just no longer visit links to Quora based on principle. I don't like to jump through unnecessary hoops.
You want users to register, because they want to register. You can hold their video hostage after the preview, and they'll just create a throwaway account, or register, and never participate or return. Is that something you want to encourage for the ego boost of a higher user count?
Anyway, that's my take on the situation. I'm always for giving users the best experience possible, and I think interrupting their video doesn't achieve that goal.