Originally Posted by LyfeSaver

I wanted to add a positive data point to this thread.

My ticket was booked through AMEX Travel, AA codeshare flights on JL as a revenue ticket in booking class D. As per the previous posts, JL or AA agents were unhelpful. As a last resort, I called AMEX Travel, and the agent was able to assign seats! She used the "requesting seats" as the language, but AA.com now properly shows the selected seats on my itinerary. (Confirmed on JL 67, JL 711, marketing with AA 8445 and 8482, respectively.)

I confirmed that these seats actually got allocated by doing a dummy booking pass on jal.com. My new seats are showing as unavailable.

While my case is for amextravel.com bookings, you may be able to try your booking agency directly to find success to get around this issue.