The problem with putting something between the lens and the camera is that you've moved the lens farther away from the sensor than it was designed to be. If your attachment doesn't have any optics in it, you've basically added an extension tube, and the lens will only be able to focus at close distances (possibly very close distances, like macro photography).
You might be able to design such an adapter that includes optical elements, basically a teleconverter (ideally, with 1x magnification, like a system of relay lenses). The problem is that any added optics have the potential to degrade the image, and there will be a cost (possibly substantial) associated with the optics. It is likely that there isn't much of a market for such a device—phototogrpahers willing to pay for it would probably just buy a body with an AF motor, or lenses with motors, instead.
On a tangent: When Pentax introduced AF cameras (which had built-in motors), they also released a 1.7x teleconverter that could provide autofocus capabilities with manual-focus lenses. The camera motor moved components of teleconverter, altering the focus of the lens. The range of autofocus was limited, so the MF lens had to be manually focused to approximately the right distance first.