Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.601 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
Use of the BA companion certificates would require flights on BA metal only -- with the concomitant hefty surcharges. If you are willing to pay the surcharges, I would call BAEC and ask about an open-jaw routing on your certificates: NYC-LHR (stopover) - FCO//PAR-LON (connection) - NYC. You would have to get yourself from ROM to PAR on your own.
Note that finding low-tier TATL award space in the Summer can be quite challenging.