What I am trying to convey, is that with no dampers the weight is exactly proportional to the height of the end in question.I'm not exactly sure what you are saying by "With no dampers the weight is on until the car rocks forward in pitch." ??? I'm also not sure how what you have typed is essentially any different that what I typed.
Anyway, it's been my experience that adding rear rebound keeps the car more stable under trail braking... in all the cars I've ever driven in anger. David, I'm confused how it seemed to have no effect or make it worse. Am I correct in remembering that you are running 500/800 on street tires? Yowza. That probably makes it very difficult to tell what your damper settings are doing at all.
So when you first jam on the brakes there is no lightening of the rear or increase in weight on the front until after the car has pitched forwards about the pitch axis.
On the other hand the rebound (or compression) has zero effect in steady-state braking. When one jams on the brakes the dampers are reacting to the velocity in pitch, which is a velocity in compression on the fronts and in rebound for the rears. To little front compression and the front should lock because there is not enough downwards force. To much rear rebound and the rears will get too light (lighter than they would in steady-state braking) and the rears will lock.
In general adding rear rebound will not help to keep the rears from locking, and too much will make the rear go light. (Maybe too little rear rebound might prevent weight transfer to the front?? Or if the pads have a high initial bite, which diminishes then less rear rebound might also help. Lastly a lot of front compression and little rear rebound would make the car raise at the rear and stay the same at the front, so the center of mass would move up. That s clearly ideal for maximising initial braking, but ignores all the other duties of the dampers such as controlling chassis and wheel modes as well as cornering)
So David's observation that it made no difference or made is worse is exactly what I would expect.