Isn't his already happening to some extent? As far as I know, there are quite a number of North Korean refugees in China, and some already made it to embassies and the like.
Would China decide to end their policy of catching them and sending them back - or at least take a blind eye to what is happening - I guess that we would see a situation where many more North Koreans leave.
Another possibility is the South "buying" North Koreans out from prison camps and the like. West Germany did that to some extent, and North Korea is desperate enough for cash to consider that. That way South Korea could control the exodus to some extent and the North Koreans, being liberated from camps, would be more welcome and easier to integrate, I guess.
Ironically, 20% of the population leaving might actually stabilize the system.