Hi John,
Although a couple of brands advertise this feature, most models of windvane self-steering do not serve as an emergency rudder. The servo-pendulum or 'oar' is there to provide the force to turn the main rudder; it does not have enough area to steer the boat. But a windvane installation like mine does provide redundancy for the all the steering gear - wheel, binnacle, cables etc. - since it acts directly on the quadrant (or, in our case, the Edson radial drive wheel). If I broke a steering cable, I would rather use my windvane to steer than wrestle with the emergency tiller. I started a thread on my windvane installation, with pictures, back in 2012 - you can access it
here.
The maker of my vane, Cape Horn, does offer an '
emergency rudder kit' that is added to part of the existing windvane structure. I think my preference for emergency steering at sea would be more along the lines of towed warps or drogues, but I haven't really researched it as of yet.