I, unfortunately, haven't been able to find set photos of them wearing the gaskets, though I did find a never before seen Dark Trooper pre-CGI photo from a VFX reel:
Also in this video from the Lucasfilm Showcase panel at Star Wars celebration (8:39), Jon Favreau says that everything came from the Mandalorian storage, sets, and archive, so that could at least add credibility that they did exist on set and were not added on as a part of the exhibit right? Also, after viewing the Hybrid VFX videos, they handled all the Dark Trooper CGI shots, one of them that was absent was the Dark trooper grabbing Grogu. I believe the hands we see on the Exhibit were close-ups used to physically grab Grogu in front of the camera.