The lift pressure is dependent on the distance from the spray root, and the angle of attack (Tau in fig 2).
Qualitatively, if you have negative rocker in the middle of the board, the angle of attack will decrease for the red area, and increase for the orange/yellow areas.
Given that everything else stays the same, the area under the curve stays the same, but the peak moves more to the rear of the board.
I've drawn in a qualitative indication of what I expect the lift force distribution for a negative centre rocker looks like with the green line.
That's my story and I'm sticking to it