I was thinking about this and came to the same conclusion. If there is any freeplay in the sprocket to the shaft, it will loosen up the bolt. the sprocket has to have freeplay, otherwise you will need to press fit the sprocket on the shaft.

Your best bet would be a shoulder bolt and use a warpy (Bellville) washer to reduce the amount of freeplay. Let the shoulder of the bolt bottom on the end of the shaft. Another method would be to make a collar that the bolt tightens down on, but does not transmit the torque to the plate. Use Loctite on the bolt threads, and a touch of grease on the bellville.

The Ducati manual suggests a thick grease on the shaft splines before you install the sprocket. As I think of this, Disc Brake Quieting grease would probably be the ideal stuff to put there. Its super thick, doesn't wander, and has a damping affect.

Cheers,