Look into the Oneworld Circle Atlantic fare, you should be able to route something like this and squeeze into the under 21,000 miles fare:
EZE-MIA-SFO, SFO-LHR-FRA, FRA-LHR-EZE
Details on the fare rules are available on the
Oneworld website , but you'll need a travel agent or probably even AA to help you price this in Business.
I have no experience with this multi-continent fare, so don't even know whether it would be of good value to you.