Where should I attach the rod to the aircraft model when simulating wind gust /weather vane effect in wind tunnel testing? At the center of gravity, behind the CG, in front of the CG...?
In order to measure yaw derivatives, the airplane has to yaw when in the tunnel. If you mount it at the center of gravity, which should be reasonably close to the geometric center, it will stay in the center of the tunnel section.
In the old times, windtunnel models were mounted on strings. I expect that your model sits on a long sting. This sting sits on a rotating scale which rotates such that the mounting point does not move around when the scale is rotated. Alternatively, the scale is inside the model (see picture below). If your arrangement is different, please point this out in an edit of the question!
Internal force balance and sting (picture source)
So you want to know whether a given aircraft will initially yaw to the left or to the right when struck by a sudden gust of wind from the left. Where should the pivot point be in a wind-tunnel model?
It should be at the C.G., assuming that you are modeling the aircraft in flight and not in some other configuration such as rolling along with some portion of the landing gear in contact with the ground.