SilverWook said:
Skimming through my yellowed copy of Once Upon a Galaxy, the only mention of Kurtz in the post production phase is in the November 2nd 1979 entry.
Author Alan Arnold went to Kurtz's Sausalito home with Irvin Kershner to pilfer records to use as a temp track for the ESB score. Kurtz was out of town at the time.
Post production is the most brief part of the book though.
Yeah, Rinzler's book cover much more of the post production than Alan Arnold's journal but I don't recall the details. But like Kazanjian mentioned he was still very much involved - present for the mixing and timing etc. He's also present on the spotting sessions with Williams as seen in the BBC documentary on John Williams, and there's this interview: http://www.ign.com/articles/2002/11/11/an-interview-with-gary-kurtz?page=4
IGNFF: Do you think that he felt he'd outgrown the need for a set of controls... A "no-man"?
KURTZ: I don't know. I don't think we ever talked about it in those terms, but I think that he did chafe a bit under the idea of someone saying "that's not a good idea," some of the time. At the very end of Empire ... we decided at the very last minute – we pretty much locked the picture in the mix and just getting ready to make 70mm prints – and we decided that there had to be an extra shot at the very end, to identify this rebel fleet.
If you remember how the end works, it's before you go into the medical department, who are working on Mark's hand. It's the establishing shot of the fleet, and we had a shot already of going into the window and showing Mark inside, and we just decided that it was confusing We didn't know exactly how that was sorted out, so we wanted a long shot at the beginning, and then one at the end that shows the whole fleet when the Falcon flies off. They weren't very difficult to do, and all the ships were there ... just pile up the composites, and they were rushed through, just to get it done. Very last minute. One of them wasn't particularly good, and George said, "Oh well, maybe we should just let it go."
I said, "It's worth at least one more go through. One bad shot can ruin the whole movie, basically." Which I really believe is true, and it just wasn't very good. It was just a compositing problem, had nothing to do with the individual shot elements – I can't even remember what shot it was, now. I think making a movie wears everybody down. You have to be really careful of the decisions you make at the very end, because you can kind of throw a monkey wrench in, very easily.
Going by this, he doesn't seem to be someone that's been seriously cut off from the production. He is also the one doing all the promotion and interviews at the time of release, not Kazanjian. See the Japan release for just one example.