Unfortunately us PP holders are out of luck in ICN T2 - SPC, Lounge L, and Matina all don't have showers. (Matina was my favorite in ICN T1.)

I've been burning lounge passes (from KE credit card and from Morning Calm) to use the KE lounges (just to shower).

There are free public showers (one on the 4th floor, near gate 231) that are supposed to be free for transit passengers - not sure about regular departures.