Hi,
hmm, I am not sure what you are trying to do. So first of all, there is no Mat Shader in TG2. Do you mean the default shader(s) in the internal network of objects?
In this, you can indeed set the object part's (!) specularity. This specularity is "global" for this object part. You can furthermore modify this specularity with a reflectivity image or function, and reflectivity roughness image or function.
Hope this helps, but I'm afraid you may have meant something else all along.
Frank