Basically this is the original way of booking a 241 return and the OP will need to call a BAEC service centre (Japan probably) at midnight to book. The advantage is as already noted that he will have surcharges assessed as if it is a GIB return.

If the OP just books a separate inbound flight online at midnight and then calls in the morning to have it added to the 241 then he will be stuck with with surcharges ex-GIB for the outbound and ex-US for the inbound.