Well, I’d add bookkeeping code to the program. Whenever you create a vtkActor/vtkMapper alongside a vtkShader in your scene-building code, you could add the two objects to some indexed STL container (using the vtkShader pointers as key). So, when you get a vtkShader via GetVertexShader(), GetFragmentShader() or GetGeometryShader() via vtkShaderProgram in your callback, simply query the index to find the corresponding vtkActor/vtkMapper. I know it’s dirty, but I couldn’t think of a better way to do what you want.
Edit: It’s not Python, it’s C++.