I would like to use a Nikon D80 body with a reflecting telescope as the lens to take pictures of bright/textured objects in the night sky (e.g. the moon). The D80 has autofocus capabilities; the telescope does not. From various web searches, I've become aware of people who have fitted motors to their telescope lenses to allow for electronic focusing. However, my question is: if I was able to intercept the SLR body's signals sent to the "lens" is there a standard by which I could interpret them into focal plane movements that could be used to drive the focus motor?
Yes and no. It's possible, but there is no standard. The lens mount communication protocol for each brand is a proprietary secret. The closest to open is the Four Thirds system, which is still secret but you can buy into it. For everything else, reverse engineering is the only option. (That's how lens makers like Sigma produce AF lenses for other systems.)
It's possible that some DIY reverse-engineering work has been published online, but I'm not aware of any.
I know this is just an idea, but if you get your hand on a broken lens that is compatible with your camera and that still has an active motor, or at least an active driver chip, you may be able to rather intercept the signals that are sent to the motor, that may be easier to reverse engineer.
Consider that there may be lenses that work with just DC motors, Servo motors or piezoelectric motors (I don't know is Ultrasonic motors are a kind on themselves or fit into the piezoelectric category).
By "broken" I mean a lens that has lost its glass, or that has a broken transmission or somehing like that that is still electrically fine.