So if you want to reserve your meal in advance (i.e. before check-in), here's a little trick I figured out.

1) go to the reservation page on AA.com. The url should look something like this: https://www.aa.com/reservation/selec...Locator=XXXXXX

2) once on the page, paste the following link into the URL box and hit enter: https://www.aa.com/reservation/checkinConfirmation.do

3) the page will reload and ask you if you are travelling with infants. Ignore the error message. Just click "no" and then "continue".

4) On the next page, scroll down and select your meals. Even though you can't check in yet, your meal selection will be saved.