Have you been to
Seoul Metro's website? While their home page shows a pop up for the Seoul Subway app, i find the main webpage easier to use.
Go to "Station Info". A map of the subway network shows up. Scroll down the page, it is located in the "Cyber Station" box.
Select a station
Select "Information"
Select "Fixtures". Will show a link for "Guide map for inside". Select that link.
It clearly shows where all the elevators are in a 2D and 3D format.
EDIT: I just checked Myeong-dong. The fixtures shows "Sorry, coming soon." Bummer.