Data point
Booked AA codeshare / JAL 005/033 (JFK-HND-BKK) in Y & called up AA's customer service no. in Asia
Was quite surprised that the call center is based in Malaysia so it was absolutely easy to talk to the agent & he did everything he could to help me select my seats
Since the flight is operated by another airline, the only extra legroom seats I could get (as OWE - higher status opens up more seat availability) were the bulkhead rows
The EmEx seats are blocked.
I used SeatGuru for seat map advice but found that the 772 aircraft maps (for HND-BKK) are not updated/correct so I referred to the JAL website instead
For JFK HND, SeatGuru 773 maps are correct.
I also have the JAL PNR but it doesn't work (it won't work) on the JAL website, I've confirmed that with the JAL customer service agent (they are based in HKG)
This thread was very helpful so thanks to all who contributed ^