Well, just to follow up and close the loop here, AA finally posted the miles for this flight. At the end of December, someone finally respond to the missing miles request and asked for more documentation to be sent in. I provided the boarding pass, itinerary, a letter from Fiji saying the flights were taken and should be credited, and a letter from AS saying that they denied mileage credit for this flight because it was a codeshare. That seems to have been sufficient for the AA agent that reached out to then post the miles. Of course, it was a month too late to extend my mileage expiration date (Had to do a donation to keep them alive.)

When I reached out to Fiji Airways, all they would provide is a letter saying the flights were taken and requesting whomever we sent it to credit miles -- they didn't seem interested/able in actually doing anything on the backend to push the miles to AA. But in the end, it all worked out, so I'm happy.