I think you are on track, and you have found the following code in ninjaVideoPlugin.js:
f.className="stopVideoIcon";f.onclick=function(a){g.unloadPlayer();a.stopPropagation()};
Then you can add your own function into the f.onclick event:
f.className="stopVideoIcon";
f.onclick=function(a){
if (myStopVideoCallback) myStopVideoCallback();
g.unloadPlayer();a.stopPropagation()};
And add the following script to your page:
<script>
function myStopVideoCallback() {
$("#ninja-slider-prev").show();
$("#ninja-slider-next").show();
}
</script>