Creating an alpha for Matinee output: how to edit Engine Materials?

I’m interested in extending the outputs of Matinee (e.g. by adding a user-defined alpha), and I can see that HighResScreenshot already does some of what I require. It achieves this by applying materials such as:


The trouble is that I have no idea what this material is doing. I’d really like to be able to inspect that material, so that I can build my own extensions. Is it possible in general to inspect engine materials? I presume it’s not correct to try to import them into a project?