Maybe the Sarek/Chapel pic was part of a clip from 'Entertainment Tonight' - not from the film itself, but from the show filming the production-in-progress. I seem to recall they did some behind-the-scenes bits on the making of TVH in early-mid '86.
I found this one:
Entertainment Tonight on "Star Trek IV, the Voyage Home" - 1986!
But no sign of Sarek/Chapel scene.
However you are absolutely right. The screengrab could come from a special or similar.
By the way, has anybody seen this:
ET Flashback Star Trek II The Wrath of Khan
Am I wrong or this is the first and only behind the scenes footage ever released on Star Trek II?