We have done many timber frames in Nevada, Washoe and Placer counties and always use the exterior enclosure system as the lateral load resisting system. We use the frame only for gravity and uplift resistance
It is my understanding from taking a course from Dick Schmidt and Rob MacKay and working with other structural engineers, that the timber frame does not have rigid enough joinery to be considered a moment frame, nor enough bracing to be considered a braced frame.
There is an article in the new Timberframing magazine about larteral resistance. I haven't read it yey, but it might address your concerns.
I'd love to hear others opinions.