That's a good point; the laws in Korea would make it a felony to use a false SSN for identification purposes. Not sure they would actually try to extradite a foreigner to Korea to face charges, but as it's a felony, they probably would be within treaty rights to demand that if they chose to.
Of couse, a real Korean with a real Korean SSN would be fine ;) However most of the people on these forums probably don't fit that bill.
In any event, I doubt the keys would work cross-nationality; I guess try it and find out, if you have a real Korean SSN to try with. I don't. Worst that can happen is it doesn't work.
Actually for that matter does actual NA codes from the game (if you buy it) work in other NCSoft Lineage 2 regions and does NA Time cards work in Korea for instance?