ActiViz only exposes the public interface of VTK classes. The protected methods cannot be accessed so there are no huge benefits deriving VTK classes in ActiViz.
If you want to add observers on your VTK objects in ActiViz, you should have a look at event handlers instead of vtkCommand.
For instance you can observe the EndInteractionEvent of a widget using the following:
boxWidget.EndInteractionEvt += new vtkObject.vtkObjectEventHandler(OnEndInteraction);
And define the corresponding callback like this:
public static void SelectPolygons(vtkObject sender, vtkObjectEventArgs e)