No you wont be able to get SQ boarding pass in NRT since it is a separate ticket. If it was same ticket it would be no problem. You could try visiting the other NRT terminal first to check in with SQ (may or may not let you) then swap terminals for OZ check in.

But easiest is to get SQ BP at ICN. You will need to show transfer security your onward ticket. IIRC transfer desk will be closed by the time you arrive and not reopen until 7:30am. There always seems to be a big queue at that time. Last time the helpful ladies in the SQ lounge got my onward BPs for me while I enjoyed the nice lounge.