The "generic starfield/cosmos footage from the beginning of the movie" is original, I believe. Actually, a fairly high quality version of it exists on DVD. A heavily truncated version of Robotech the Movie exists on A&E Home Video's Robotech: The Complete Series. Basically, it's a 30 or so minute version of the film with all of the Megazone footage cut out. You might check it out.
I hope that helps. Good luck.