there is a transit hotel inside security at ICN, which should make it a lot easier.
ICN Transit Hotel
The hotel appears to be airside, making it quite convenient for people with a few hours layover.
I'm working on an ex-ICN DONE4, and separate positioning flights to get to ICN, which don't work as a connection, so we'll have to overnight in Seoul. I assume we'd have to check bags only to ICN for the positioning flights. So, when we get to ICN, I guess we clear immigration, collect the bags, clear customs, but then how do we get to the hotel? Being the night before, it's likely too early to check in for the DONE4.