On my most recent trip booked as AA codeshare, AA's link to OLCI finally worked.
Trying the old way (get JAL PNR, go to JAL's OLCI, input info there) had never worked for me.
Once in to the JAL OLCI, you can work part-way through the process, get to seat selection, and once selected, drop without completing OLCI (if you don't want to - I figure I'll be at the business class counter at the airport anyway). JAL will save your seat selection.
This also allowed immediate seat selection for the return leg even though that was far more than 72 hours away.