I am trying to create Matte/Shadow Objects for rendering with Karma.
I am not trying to render AOV shadow passes. I need complex matte geometry to receive shadows, as it is possible in, well, any other 3D app.
MaterialX has no matte/shadow shader and rendering different passes and combining them in a composite is not the solution I am looking for. I need to render everything in one go.

So I am using the Mantra matte/shadow shader, which seems to work on the shader side with Karma, (Although I have no way to check the alpha channel in the Karma render viewport for some reason). But now the problem is that the matte object is being reflected in the scene and none of the render visibility settings seems to change this. Also I need to control shadow casting and self-shadowing and I do not see any options or settings to do this. As I said, I can not even check the alpha to see if the matte object is casting shadows on itself.
What am I missing here? Any help appreciated.

Yes well, another year of Redshift then, until Karma goes from production ready to artist friendly

I think you might want what is in the render geometry settings lop perhaps? For Karma you can set the holdout mode there for the geometry as None, Matte or Background.