I booked two F seats on NRT-JFK for June

Back story -

Had CGK-NRT-BOS booked in Biz
Needed to change to CGK-NRT-JFK, but looking for a new reservation only showed a mixed itin with Biz/Coach
Used the online tool, asked to change the two segments instead:
CGK-NRT - same date
NRT-JFK - next day

Came back with a mixed itin, with NRT-JFK in FIRST (A) and to my delight, it actually confirmed and was able to select seats on JAL.

So, it IS possible!