I remember reading an interview with a writer by the name of Carrie Fisher a few years back. She's had a couple of novels published and works mainly in TV now but she originally started out as an actress (I guess being the daughter of Debbie Reynolds and Eddie Fisher opens a few doors.)
I can't remember exactly where the interview was but she said that she played one of the leads, some sort of.... space princess rebelling against the evil space Roman empire? Though she described the screenplay as "fantastic" and "wonderful," the production went off the rails from day one. Lucas was completely overwhelmed, the special effects that were supposed to be revolutionary never worked and - surprise! - the British crew in London where they were shooting hated him.
So yeah, I think if it had ever been released, it would be a completely forgotten piece of dreck by now. Alec Guinness, the one name in a cast full of unknowns (then and now) always said that he loved it, that children all over the world should have had a chance to see it, but I can't see it. The big sci-fi revival in the 80s was always going to come from TV and The Star Wars wasn't going to derail that (though butterflying away those eight Space 1999 films would've been manna from heaven.)