With V-Ray 3 and the new online licensing the demo install is the exact same exe as the full version. We just limit the number of days you can use it via online licensing.
The V-Ray material format is XML, you can edit the XML in a file, and then load and apply the material with the current interface. The two functions you would use are:
bool LoadVismat(string filename)
bool ApplyVRayMaterialToObjects(string filename, ICollection<Rhino.DocObjects.RhinoObject> objects)
That will most likely meet your needs, but it would be helpful to know more about your final goal.