Just did an AA codeshare, NRT-SIN, on JAL. Both airlines (AA & JAL) said they couldn't pre-assign seats. AA said they could only put in a request for Window or Aisle. BUT, JAL said they could ALSO put in a request for an exit row. And the JAL res. agent on the phone said those requests are "honored" on the day of the flight, at the airport, based on when the request was made, the sooner the better. So, I'd say, call the codesharing airline and get your requests in. I put the request in, and the reqeust was indeed granted. Interestingly, it was printed on boarding passes issued in STL the previous morning (routing was STL/DFW/NRT/SIN--wouldn't arrive in NRT until the next day). The AA agent at the STL AA counter said she could assign me to a different seat if I wanted, since she could see that there were several empty rows in the rear of coach.

When I boarded the JAL flight, the F/A's offered to help me "save" an empty row in the back to sleep on, while I enjoyed dining sitting in my exit row seat. (just fyi...the F/A's & overall service in Coach on that JAL codeshare absolutely surpassed by far anything I've seen in domestic first & international biz. on AA)