Since vtkOBBNode is a protected field in vtkOBBTree how do you intend to access it in your wrapper code?
Is the class needed only for the callback in vtkOBBTree::IntersectWithOBBTree? If so I suggest you redefine the callback passing double Corner[3] and double Axes[3][3] parameters for each node instead. Then overload the public methods of vtkOBBTree which require vtkOBBNode with the same parameters.