Ex3mx helped me test this with his latest map, so I can now confirm you can make your own materials for your levels. This is something Ex3mx did in his map "Museum" for the coloured bits, if you want to see an example of what you can do with this.
You basically just have to send me the file for the material too when you send the map, and they work fine together on the other end.

I'm actually almost certain that the same process will work for static meshes, if you wanted to create a custom mesh in a 3d modelling program and import it into unreal. This is something I mention in the guide.

There's plenty of information on actually making materials available online, so that's not something I've gone into.

I am still planning to make a video on this and other topics when I get the chance, since I know some people prefer videos. I might touch on the basics of material creation in a video as well.