Hmm - I just tried to do this for OZ105 NRT-ICN (which is sold as a 2-class flight, but is scheduled as a 747M) but they wouldn't let me.
First, I was told that those rows were only for first class passengers, but when I pointed out that first class tickets were not available for that flight, she responded that they are not open for business class and fly empty. I pushed that business class ticket holders have used row 1-4 on a 2-class flight before and was told that they will open only on "rare" occasions.
Is this because it is a 747-combi? Or does one have to wait until checking-in to get these assigned? Or should I just try another agent?