I had a look at the way WeatherOverlay creates its material, which is the MatLoader class, which requires a "path" parameter. It then appends this path to "Materials/" and casts an Object created by Resources.Load(appended_path) to a Material.
I guess my question is now, where is the "Materials/"? Is it an actual folder, or is this just a pseudo-folder system that Unity uses?
Anyways, I'll try looking at the original assets with some sort of program like you said, thanks for the suggestion.
Edit0: couldn't seem to find any legit ways to look into Unity assets.
Edit1: also just noticed that the "moveSpeedMultiplier" in WeatherDefs doesn't seem to have any effect. Is this deprecated? Edit2: I'm stupid, it does work