FWIW, I've done CGK as a mileage run twice, with 3 of those segments being the NRT-CGK and return on JL. (one time I arrived via HKG) I'm 2 for 3 on getting moved to an empty row on those flights, resulting in 'lie-flat economy class.' The JL agents at check in or the lounge have treated me well as a OW elite. The one segment I didn't get 3 to myself, I still had no one in the middle seat.
They also will put you in premium econ for $200.