If your luggage is tagged to the final destination, then AA will transport it there eventually. If you have a missed connection at the hub, your luggage will NOT be loaded onto the connecting flight (because 100% positive bag matching is required on international flights). If your luggage is mishandled and delayed, AA will take maintain custody of the luggage through customs clearance as necessary.

I doubt that you would be able to retrieve your luggage at MIA if you misconnect, since the luggage will remain in the bowels of AA's operation. It will be moved to your new flight to BCN (presumably connecting through LHR or MAD, or perhaps the next day's nonstop flight to BCN). Of course, if you change your mind and decide not to travel, AA will release your luggage to you.