AA codeshare has always been a challenge. Some rumours of success by getting the JAL record locator from AA and then contacting JAL. Personally, I have never been able to assign seats on AA codeshare if the TA did not assign them at booking.

Mid March and a couple of weeks could be busy as Japanese companies tend to rotate their expats for fiscal year end. But as mentioned with the present arrival caps, the flights are unlikely to fill up.