I can concur with Ethelman that this will be no problem - I have done it numerous times. Having said that, I did get questioned recently by a non Asiana check in agent, but I checked with Asiana in Seoul and they said it was fine. See this thread:

http://www.flyertalk.com/forum/miles...-wrong-me.html

Good choice to stay downtown Seoul too, far more going on than at the soulless (no pun intended) airport hotels. As you'll only have hand luggage you'll be ok taking the train downtown too.