I misread the post above about the pipe. It would be nice if everything was staright forward. The change in momentum could be used to fin the force of lift on the entire stab if you knew all of the variables. I think it would be nearly impossible to calculate the load on the control surface itself off of the force of lift off of the entire stab.

Banktoturn is right. There is no particle movement across or perpendicular to the streamlines. The velocity along a streamline is not constant. We can consider the flow we are dealing with as incompressible with little error. A higher velocity can be noted from the streamlines being closer together because mass is conserved and density remains nearly constant. To get more mass through the same area the velocity must increase. You can kind of see this on your plot. On the upper surface of the airfoil the lines are stacked closer together while on the bottom they are further apart. The higher velocity yields a lower static pressure and the oppisite for lower velocity. Thus we have a pressure differential and lift.

Also if we are going for pure accuracy, a 3D model needs to be used.