In answer to PP7's question.
First, my money that day was on King Brian, obviously a well-named conveyance and coming into the race with three wins on the trot including the Rosehill Guineas. It finished third, with a close up view of the shenanigans up ahead of him,
Second, some back of the envelop calculations.
Suppose the horses were dead level with 50m to go; they would haver been travelling at, in round figures, some 20m/s. So they had about 2.5s to go, The relevant bit of physics that describes the effects of the leg-pull is that the impetus (the product of the applied force and the time interval during which it was applied) equals the change in momentum (the product of the mass and the change in the speed).
You've supplied a value for the mass, 500kg,
The change in speed (Blue Era goes a little faster, Summer Fair a little slower) can be worked out as follows: Blue Era finishes half-a-head in front, that's about 10cm. So he has to gain 5cm in the last 2.5s while Summer Fair loses 5cm (one horse is being pulled forward, while the other is being pulled back); a change in speed of 0.05m/2.5s, that is 0.02m/s. That's a change in momentum of 500kg x 0.02m/s. That's 10kg.m/s.
That's also the value of the impetus - the applied force multiplied by the time interval during which the force is applied.
So if Schumacher pulls Hills's leg for 1/2 a second, the force he applies is some 20N, the same as the weight of a 2kg block of ice-cream (or butter or anything at all).
As I wrote, back of the envelop stuff, but I reckon it would be between 10N and 50N.