Stretch second texture within Shader

I got some terrain mesh built with blender. The UVs are set up to repeat a texture about 10 times. The mesh is texture mapped.

Now I want to blend in some second texture onto the mesh surface, but it should be stretched all over the mesh without repeating or clamp. I had the idea of writing a shader taking a second texture, but I guess I'd need some second UV map. I read in the Unity forum that Unity supports a second UV map, but is normally dedicated to lightmaps.

Any way to achieve that? Thank you