Interestingly I have just come across the following on the Korean Air website:
NOTE
Passengers who fly on an international flight and connect to a domestic flight in Incheon Airport must claim their baggage first due to entry and customs clearance in Incheon Airport and complete separate check-in at the check-in counter for the domestic flight.
So it seems that even *if* BA would combine on the same PNR a reward 2-4-1 reservation with a cash reservation, or check a bag through to a connection with a carrier outside One World, in this particular case we would still have to collect baggage and check in again due to Korean regulations. Making my original question a moot point
As a FYI for anyone who finds themselves in this situation, the minimum recommended time at Incheon to do this is 80 minutes. However, this figure is quoted on the Korean Air website, and thus assumes you are connecting between an international and a domestic Korean Air flight. Since we will be connecting from British Airways international to Korean Air domestic I am going to leave closer to 120 minutes because this will necessitate a terminal change at Incheon as well (which a Korean Air to Korean Air connection would not).