Here's a few things I think you should all know:
1. The shelter at Biseondae holds about 80 people and according to Korean climbing friends should be fairly empty over Chuseok. The shelter has a convenience store and fairly decent restaurant, good gamjajeon and bibimbap.
2. Anybody expecting to do easy single pitch sports climbs from there is going to be sorely dissapointed. There's only big, commiting multi pitches, that require trad gear. It's not even possible to just do the first pitches as a lot of them are 35m-40m and lack decent protection.
3. There are heaps of boulders around there, so if anybody has a pad, that's an option.
4. The only place in Seoraksan with sport climbing is Agulbawi (sp,) but it's nowhere near the shelter and is a ballache to get to. Info here:
Having said all that, it's a beautiful spot and the hiking, bouldering, river swimming and company should make it a great time.