Question Can I define the shape of a mask with a material in a Scroll View game object?

Hey

I was just curious if it's possible to define a mask in the "Viewport" game object with a material, instead of a texture/sprite. I tested it with a shader material I created (a star with a transparent background). And it doesn't seem to work. So I'm assuming UGUI doesn't support it, but just asking in case my assumption is wrong.

(for clarity the Viewport game object I'm referring to is a child of the Scroll View game object you can create from the UI menu.)