Laying a texture over an entire terrain

I'm working on a grand strategy game, and I want to have a realistic looking (Unity standard) terrain that I can optionally cover with info layers, which will color parts of the terrain: region borders, ownership colors, location names, etc. Essentially, I want to use the normal terrain texture engine with splat maps as-is, but also have a high-res extra texture on top of them that I can replace on the fly, and apply with alpha to the entire map.

What's my best approach here?

Should I try to add this as another terrain texture, and just tweak the splatmaps dynamically? That seems like it would give me trouble with alpha, like if a lot of that texture is clear, and I'm not sure how good it would be with a very large texture.

Should I try replacing the default terrain material, and add another shader to add that extra texture? I'm having some trouble with that--after over an hour of frustrated searching, I've been able to download the built-in shaders but I can't find the default materials to copy and change.