OP - please clarify. Are you US elite? Or AA elite?

If you're AA Platinum, flying on AA, you should have stickers, so I don't understand what's going on here.

But it sounds like you're US Elite, flying on AA. See the Wiki. Open F seats at check-in, should be first come first serve to US Elites on AA operated flights - just as AA elites can upgrade on US flights.

Not sure why all of the other posters focused solely on "tricks" when OP might have been asking about the horribly-explained cross-member benefits of AA/US pre-merger.