I recall this coming up in the past - the suggestion that some Vulcans in the background of STIII were mannequins. But I believe someone posted BTS photos showing they were actually actors? Or did I hallucinate the whole thing?
It doesn't surprise me that someone else may have thought the same thing. I did try googling it before posting and did not find anything. But I just searched the boards for "scream mask" and see now that someone bought it up back in 2010.
They were all actors, including George Takei, who pleaded to be a Vulcan extra at the night shoot on the steps of Mount Selaya! The courtroom scene in ST VI, however, was populated with extras in over-the-head latex masks and, in the background, supplemented by little (ERTL?) Klingon action figures!
I did see a photo somewhere, of all the little figures lined up. Maybe they used ERTL's ST III Klingon Leaders (Kruge). (Or, perhaps Playmates already had the 1992 Gowron figure in production, but as-yet-unreleased)?