trythis, I do see where you are coming from here, but there is a key point that you have not taken into consideration - the amount of force or energy needed to get different materials to vibrate. You mention rosewood xylophone keys. To get one of these to vibrate enough to produce a good sound you have to hit it quite hard, relatively speaking of course. Vibrating guitar strings just don't work with this much energy, and you wouldn't get much vibration or sound volume from a guitar with a rosewood front. But spruce vibrates readily in response to the small amount of energy being transmitted through the bridge. And if you had a spruce camera this could easily vibrate from the low amount of energy given off when the shutter is fired. A rosewod camera would probably vibrate too, but to a much smaller degree.
Alan