I believe the transport infrastructure across the N Korean and China border, combined with the social and political controls, allows the border to be sealed pretty effectively against significant numbers moving north.
Do you really think the Korean leadership is going to allow it's population to demonstrate to the world a western embargo has any effect on them ? What do you think they import from the west now that will bring them to their knees in any short term scenario ? It's a cult dictatorship, people are prepared to die for the 'leader'.
Secondly China has the capacity to support N Korea as much, or as little, as they decide. Another benefit of having huge manpower resources used to rigid, (but much less so than in the past), central control.
I'm not convinced a western embargo would significantly change the situation unless China and Russia join in.
