These cruise missiles would certainly be flying "nap of the earth" violating NK airspace and could be shot down legitimately.  I suspect NK does have defensive countermeasures effective against subsonic cruise missiles.

Now what about NK's latest test -- the one that overflew Japan, sparking such outrage?

When NK launched its IRBMs a couple months ago on those lofted trajectories, the media made a big deal about how they flew higher than the ISS' orbit.  Well, so did this one.  It overflew Japan at roughly its apogee of 340 miles.  That obviously put it well above Japanese airspace.  Its trajectory also made it clear that Japan was in no way being targeted.  It was an imprudent provocation, but one they're basically entitled to conduct.

Your proposed one?  Not so much. 

"If any vessels are in the landing areas, it would be unfortunate but acceptable collateral damage."   Sink a Chinese ship and then go tell that to Premier Li.

Put down the Tom Clancy novel and back away slowly...