Maybe you know all this already but...
If you get a chance, go riding in Gangwon Province in the area around Gapyeong, Gangchon, Chuncheon, Hwacheon, and Hongcheon. The climbing can be daunting but the roads are great and the landscape is spectacular. I did a lot of riding in this area in 1996-1998.
There's even a velodrome in Chuncheon! When I was there a high school in Gapyeong had a cycling team and I'd see them training all the time. Sometimes guys from Seoul would ride out our way as well.
In general there are a lot of good roads in rural Korea. The quality of the pavement is excellent and not so much traffic. Do you ride with any Korean bike clubs?