I'd recommend avoiding classes specific to MultimediaViewer for this purpose.
The semantic intent here is to mark images that are not considered part of regular article content. These would be presentational elements like user interface icons, images part of a larger construct (such as clipped images, map pins etc.). It's not at all related to MultimediaViewer and is useful for other tools as well. Don't forget that what MMV is doing is by no means new. Gadgets like these have existed for years and people will continue to use and develop these. This is good; we want people to stay inspired (and even competitive in a way). These gadgets would greatly benefit from a simple class name filter to replace their current approach (lots of exceptions for arbitrary class names, and individual patterns like "Clear crystal" icon).
Making this MMV-specific would give MMV special treatment resulting in hacks and maintenance burdens we don't want. A class like no-mmv" masks the real intent. In my experience that would discourage communication between users and developers when issues arise. Not the users that read it here, but the users that copy it further down the line; whom won't know its purpose.
Making it specific to the idea of a "viewer" (e.g. "no-viewer", "viewer-exclude", or "for-page-only") is better in my opinion, but only marginally so. I'd recommend aiming for something that reflects what it is and allows separation of concerns. Then have MMV use that in its filters. This may mean we'll need two instead of one if the types of images in this category are that different, but that would imho be a good thing.
— Krinkle