Robbie,  You wrote:

> Although the center of mass (hence the inertia) might shift slightly
> as you describe, I think the energy stored in the compliant shank
> dominates the situation.  Think of a limber golf club, or the
> pole-vaulter with a fiberglass pole.

Please don't think of a limber golf club (unless you're going to
let go of it just before you strike the ball), or a pole-vaulter with
a fiberglass pole.

The difference here is that the golf club and the fiberglass pole are
both reacting against something.  The golf club is still being driven
by the golfer as it strikes the ball.  The stored energy in the club
shank reacts against the golfer's hands as it releases its energy.
The pole-vaulter's pole is giving its energy back to the athlete
by reacting against the ground.

The hammer shank, after let-off, has nothing to react against.  It's
free-floating in space, and nothing other than gravity or friction is
going to affect the acceleration or velocity of the center of gravity
of that hammer and shank.

Ray Fairfield

 [ The shank may still be "ringing" from the key strike, like a
 [ xylophone bar, unless it's pretty lossy.  Otherwise I agree with
 [ you entirely.  -- Robbie