I was in Seoul earlier this year and wanted to ride MTB whilst there, perks of the job etc as I always try and ride whatever country I’m visiting.
In the end I got a ride out with a US guy now living in Seoul, he personally loaned me a bike and took me out to show me his local trails, and all on a Tuesday night. The guy was a legend and I was extremely grateful, wouldn’t even take some beer money for his time and efforts.
Your best bet is to sign up with MTB Korea Yahoo group, introduce yourself and ask what going on whilst your there. They are a great bunch and if you’re nice someone should be able to help out.
Next time I go I’ll be sure to stay the weekend so I can ride with the group further afield, but I had a fantastic evenings ride bombing round the urbanish trails on the shirts of Seoul finishing off with an ace decent in the dark with big lights.
Lots of great riding in South Korea, if you know where to go etc.
Good luck and have fun.