Fear not, for the antidote to atonal antics is here!
The autotune slide whistle, an invention the musical world has been crying out for, is simply an ordinary slide whistle, but with a plastic thingy attached (blue). A semi-rigid tab (green) snaps into the plastic thingy, locking the lever to concert pitch.
The tab could be turned to re-enable the portamento, or perhaps the handle could be flexible enough that a simple deflection could temporarily bring back the sweeping effect during a performance.
Unlike many of the concepts on this site, this one could quite easily be implemented in real life! Without too much difficulty!