An in-theatre recording of the 70mm mix for The Empire Strikes Back has just surfaced at last: http://originaltrilogy.com/forum/topic.cfm/ESB-70mm-Soundtrack-1980-in-theatre-recording/topic/12501/

Very interesting listening! As I'd long thought, it is much closer to the 35mm and 1993 mixes than previous speculation had indicated. None of that extra SE dialogue, just a few small differences in sound effects, and the music the same except for the last scene, which also has different ADR for Lando in addition to being edited differently. Recreating that part would be rather tough, as well as pointless without doing the video to match (obviously it would not synch to the GOUT). But since the rest of it is so close, I can confidently state that my most recent 5.1 mix sounds very much like the 70mm version for the vast majority of the movie, which I'm rather pleased about.