Ben, I think your thinking is a bit backwards here. Your analogy of how in the case of the body, the further away from the bridge the wood is, the less it plays a role in the sound of the instrument, is correct. I think you can treat the body as a discrete particle that has ossilations within it, but does not ossilate as a whole, if that makes sense. The neck, however, does ossilate as a unit. Thus, force from the strings is being sapped away to cause these ossilations. The farther the force is from the headstock, the more of a moment arm exists to allow the headstock to vibrate with less force. Thus, the farther you move from the nut, the easier it should become to vibrate the headstock. Now, there are some holes in that idea. One, is that the closer you get to the body, the more vibrations will go through the neck to the body and less to the headstock. Also, typically, as you get closer to the body, the neck gets wider and thicker, making it harder to deform. So my idea is 100% valid, but really the best any of us can do at the moment are close approximations.
Anyhow, that's my logic; it could very well be off base, who knows.
peace,
russ