Negative kickup won't cause the caster to reduce noticeably under suspension compression (no more than the car's pitch under braking would affect it) - you're confusing it with reactive caster, where there is a difference between the angle of the upper and lower wishbones.
One thing I've been wondering though...
With the large kickups run on 2wd buggies (30degrees), are we encouraging more suspension compression under braking (slower weight transfer)? Or does the extreme angle mean that a large amount of the compressive forces are tranmitted through the hinge pins and mountings, negating the effect of the kickup?