You can find details of the free baggage allowance for
oneworld elites here:
http://www.aa.com/i18n/utility/baggageExceptions.jsp
(that snippet is linked from the larger Baggage Allowance page
here)
The relevant portion reads:
The following passengers will be allowed a 1st and/or 2nd checked bag at no additional charge provided it falls within the size and weight limitations.
<snip>
oneworld alliance Emerald, Sapphire or Ruby members
Customers flying on the same reservation as an American Airlines AAdvantage Executive Platinum, AAdvantage Platinum or AAdvantage Gold member or oneworld alliance Emerald, Sapphire or Ruby member regardless of frequent flyer status or fare type
As for the status not being recognised, I'm not sure how to fix that... I would say that having the Marco Polo card to hand should mitigate the issue, and if the agent doesn't know AA's own baggage fee waivers as outlined above, perhaps a reference to the website would help them along?