See this link:

http://www.miami-airport.com/baggage_international.asp

Check with AA in LHR if they can "Red Tag" the luggage? MIA is a big transit point for Latin American flights. It will be your responsibility to check if the bags appear on the belt in MIA and I would suggest you only proceed through US customs once you have retrieved your bags or alternatively a "Red Tag" has worked and your bags are not at the carousel (or on the ground nearby having been lifted off).

I have sent a query directly to the AA Facebook Team to see what answer they give. The situation is variable. AA Facebook team have sent a message to their MIA baggage handling senior management for a definitive answer on what is / not possible. They were very clear about the "Red Tag" concept - how it works in practice is another matter!