I am booked codeshare with AA and I am running into the same problem. Using the JL PNR, I can get to the point of the pulldown choice for the Japanese meal but then I get the error page. I called the US JL number and the phone person was shocked that I could even get that far. They called AA and were able to get my girlfriend gluten-free but told me they could not do anything in terms of reserving me a Japanese meal. They told me I'd have to wait until I checked in with JL.

Has anyone gotten any farther with this?
Is there a thought that I might get farther calling the JL number?

And yes I know, I know...codeshare and all...given the new AA mileage accrual methodology it works out significantly better for me this way