Thank you to those that replied above. A second F seat did not open up, so we ended up flying with my wife in F and me in J (but 2A and 5A so close to each other - she gave me her caviar, which she doesn’t eat, which for some reason distressed the cabin crew, but oh well).
For others looking for F/J availability, my experience here for JFK - Tokyo was good J availability (I booked 2 together, and 2 other seats did open up eventually), and limited F availability (1 seat opened up 10 days before departure).