Hi Eethan,
JAL does have an informal rule to block empty seats next to OW upper status in coach 72 hours prior to departure in case those are still not allocated. By experience, they do release those at airport check-in on case by case: if the other seats are all booked (Japanese sometimes don't/can't select seats in advance) or if there is a couple without assigned seats yet and row 57 already assigned by then.
All that to say, you are in a good position to get the empty seats next to yours if the flight capacity allows it.
PS: If you try allocating your seat to 57, you will note that they will release all row 48 while blocking all the row 57.
Have a safe flight to Japan !