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.

Turns out the answer was Projectors in orthographic mode with the size cranked way up.