It really depends on the airline. I've flown AA to LHR connecting to BA but I was able to check in at BA at JFK before heading to LHR the last few trips. The first time I took BA from LHR, I went to the flight connections desk at LHR where I was able to check in for the onward BA flight. The agent told me I was supposed to clear immigration, go landside to check in then come back thru security but he made an exception. I would think that ICN has some kind of flight connection center at ICN where you could check-in for the Asiana flight instead of clearing immigration but I have no experience doing this at ICN.