I looked this up the other day on the AA
website, but I can't seem to find it now, just look around, it's on there somewhere...
As I recall, the policy stated there that Admirals Club admission is granted free of charge to passengers connecting to first or business trans-pacific or trans-Atlantic international flights. Its not really advertised or stated, but the times I've done it I showed my business class trans-Pacific boarding pass at the Admirals Club desk and got in with no problems.
Individual personnel working at the desk may let you in if without the above conditions, but it's really up to them. Just ask, but don't be surprised if they turn you down. It may help to dress nice and be pleasant.