As a data point...
I am flying in on AA codeshare JL metal to JFK then connecting to CLT and asked about FSS to help with connection. The response via email was "Unfortunately, the Japan Airlines aircraft arrives in a different terminal than American Airlines and thus cannot accommodate the greet as a connection. We could start the greet at our terminal 8 at curbside and take you through our Flagship check in area and escort you through security and down to the gate."