My upcoming JL flight HND-LHR was redeemed on AA miles. Then I called AA customer service to have the FF number be changed to my AS for OW Emerald benefits.

When I went on the JL website to retrieve my reservation using the needed confirmation code, I noticed under
Passenger & contact information
Related services
Membership program, Membership number
that my AS number is there, but the program is not. I don’t know if that means it’s not in the JL system.

But I do see
Flight information
Services (seats, in-flight meals, optional extras)
Select your seat
Request in-flight meal

The OW Frequent Flyer Benefits on JL are listed at
https://www.jal.co.jp/en/oneworld/jmb_benefit.html
No preferred seating it would appear though.