Have you tried using JAL's MMB (manage my booking) page with your IB flight number (not JL flight number) and JAL's booking reference (if different from IB's)? I don't have any experience with IB codeshare, but that combination allows access to my reservation including seat map based on my OW status for my AA-ticketed reservation with AA codeshare flights only. This was not possible online before but it seems there has been some change to their system.