I wonder if Warner Bros could release the original THX1138 even if they wanted to. Maybe Lucas gained control of it when they made the deal for the rerelease (by Lucasfilm paying the bills for an extensive restoration and new effects for a movie that has never been a big catalog title)
On side note, the making of documentary on the THX dvd has an anamorphic clip of Star Wars and it's the original version (Luke looking at the sunset, before the little mountain in front of the lower sun was digitally erased). I wonder what the source was, it looked pretty good.