"(I must confess to never having tried to use a BA lounge, or any other lounge, at JFK while planning to fly out of EWR. No lounge, with the possible exception of a transplanted version of CX Wings from Hong Kong, is nice enough to get me to go to JFK if I don't have to.)"

I don't doubt that you have managed to get into AA lounges on arrival and it is possible that it is a benefit that AA extends to its members. I would bet money that you wouldn't get into QF/BA or CX departure lounges on arrival nor into one at another airport. Try it at LHR one day, their 1st lounge is much micer than the AA FL.

Dave