@popof - one exception I would probably make here, sprung mass ie. a wheel is reacting primarily vertically from the ground.
Your car is turning, your wheel is bouncing, your goal is to keep the rubber of the wheel planted such that you can corner, and the reaction of the tire bouncing off the ground must be absorbed, reduced, and the tire returned to the ground.
This to me is very similar to the ski, when the ski is closer to under the skier. That being the more upright the skier-leg-knee-hip the more similar it seems to the unsprung weight concept.
But how does this change through the various stages of the slalom skiers stance?
Essentially we have a skier - 200 lbs, we have two ski packages of identical configuration, that is same stiffness, width, length, bindings. Package A has a ski that weighs 3 pounds and 5 pound of boots, package B is 5 pounds of ski, 5 pounds of boots.
From hook up through the wakes we have a ski that is on edge (same skier so same form/edge changes etc). While on edge, we have his weight + ski weight + dynamic weight (pull from boat) transferred against the water by a ski that is on edge. Skier encounters a water imperfection midway from ball to 1st wake.
Regardless of ski weight we have quite a bit of "load" in the system, and the system is reacting against this water imperfection through a diminished ski surface area (edge). Combined with this we have a relatively rigid suspension on our system, as the skier has the legs and core engaged to combat the pull from the boat. Does this skier have the same concept of unsprung weight? Does 2 pounds of ski matter much against the weight + hundreds of pounds of load?
Same skier punches through the wakes and begins the edge change, now his body weight is not on the same lean against the line, his load is diminished, the ski is flatter as it switches edges, and the skier lacks the load for his core to be engaged on, so the suspension system is less rigid, more supple. In this stage a water imperfection is acting almost vertically through the flatter running surface of the ski, through vertically oriented legs, against the skier who is not adding the load of the tow line through his legs on the system. Now we can check that same 2 pounds? Probably meaningful?