GREP detecting »Yellow Lines«?

Unfortunately, there there doesn't seem to be any way to do so.  Some years ago, someone asked how to track instances of H&J violations by script, and the response at that time was that no, you'd need a C++ plugin to track it.  That claim was followed by a very clever script that allows you to measure your current word spacing against the default non-justified spacing. I suppose it'd be possible to then build the script out further so that you could give it the default spacing, run the script, and get a count of all lines that have a spacing percent value outside of that defined by the default spacing? 

Seems like a lot of heavy lifting, in comparison to just scrolling through the document & counting them by hand.  Do you mind if I ask why you'd need to get a count of H&J violations? Perhaps there's some other way to get to your goal.