I didn't see this in any other posts so will share my experience. I was unable to check-in for the NRT-HAN segment for an AA code-share JL operated flight. On the return (SGN-NRT) I went to the AA.com site to see if I could print the AA boarding passes. At the check-in button it directed me to the JL site where I could select seats and print my BP. Not sure if this is something new but I've never been able to issue a JL BP on an AA codeshare.
BTW - didn't do much good other than seat selection for SGN as passport control would not allow entry without a JL stamp. Counter opens 3 hours before departure and I arrived early on a domestic flight. Online BP did nothing of value for a domestic to international connection in Vietnam.