Play startup animation from video automatically at login
This is a bit hacky and there may be better ways to do this, but...
Grab yourself a copy of Quicktime 7. You'll need to source a license to upgrade it to the Pro version.

Open the movie you want to auto play and select Window > Show Movie Properties ⌘-J Select the main container and them the Presentation tab check the boxes:


Auto play movie
Enter Fullscreen
Auto close (optionally, depending on what you want)
Auto Quit (optionally, depending on what you want)




Save the movie.
In the finder get info on the movie and change it's default application (open with...) to Quicktime 7. Don't click change all... The default is Quicktime 10 which doesn't support the extra movie properties. You will also need to override the Un-identified developer warning as the movie file is now considered an app. Simply right click on the file and select open, you'll get a dialogue to allow the file to be opened.

Now add it to the Users login items: System Preference > Users and Groups. Select the Login Items tab, Click the + bottom at the bottom and select the movie.

The desktop Will load for this method, so it's not exactly what your after, but it might be an acceptable fallback.