All the pre-ESB Star Wars toys were copyrighted to Fox as well.
A snag might be if Lucas put a HS clause in the Disney deal. There is also the possibility of having to license the Jefferson Starship song. Disney might not want to pony up for it. That kind of thing has plagued The Muppet Show, which has yet to have all five seasons released on video.
There could also be actor residuals involved. Nobody was thinking of home video when the contracts were signed.
Where were you in '77?