People tend to think of end correction effects (if they think of them at all) as happening at the far end of the whistle. In fact they're more significant at the fipple end - this was discovered by the French organ builder Lefebure-Wely in the 19th century (the details are in Lord Rayleigh's book if I remember right, or maybe A.B. Wood).

One way to exploit that is by holding your finger close to the fipple. You can wobble the pitch up and down quite a lot.