It's tough to guess what AA was thinking when they said "American will share plans to make Flagship Lounges accessible to even more premium customers." My guess is that premium customers means Mint and AA/B6 elites in transcon economy. Given the recent changes to the Five Star Service / Five Star Select product, I think it could also be a way to buy FL access that's cheaper than the $650 for Five Star Select.

JetBlue has hinted that they will build their own lounges in JFK and BOS eventually. It's still unknown what they will do in T5 for customers on the JFK-LHR flight. They will probably use Plaza Premium in LHR T2. Mosaic elites will almost certainly get lounge access on JFK-LHR. AA elites may or may not; the flight is the only exception to earning AA miles on B6 flights.