While I don't have the answer to your question, here is a list of hotels in Jongno/Myeongdong/Namsan, which are roughly in the center of the city but north of the river:

THE PLAZA hotel

Grand Hyatt Seoul

Lotte Hotel

Shilla Hotel

Gangnam, center but south of the river:

Park Hyatt Seoul

Ritz-Carlton Seoul

JW Marriott Seoul

There are also luxury "hanok" (traditional korean houses) if that is your taste.
Here is an example of one: https://www.airbnb.com/rooms/517829