I don't disagree with what you are saying. And perhaps a version of that approach is to only pick workshops where the leader(s) use your camera or at least use the same manufacturer and generation of technology.
There are some types of workshops that place greater emphasis on gear than others. Birds in flight, video, and night photography might emphasize the camera body. Macro might emphasize the lens and accessories. Portraits might emphasize lighting and post processing tools but not care too much about the camera body.
One of the techniques used with some groups is to have multiple instructors that use different systems. It makes for a larger group of students, but ensures a high level of relevant gear knowledge.
The DxOMark idea is a good one - especially if you have objective criteria. Even something as simple as an Amazon type rating system could be very useful.