You can optify it by putting it in /opt/ and making a symlink. It's pretty straightforward.

copy movie file to /opt/

cd /usr/share/hildon-welcome/media/
ln -s /opt/(name of video file) (name of video file)

ls -l
check to make sure you see the link in the listing

example
ln -s /opt/matrix.avi matrix.avi

Be careful not to delete the movie file in opt. I don't know what happens if it fails to find a boot video... hopefully nothing at all, but i don't want to test.