Mr A,
I have no idea what BALPA thinks of Back-2-Back trips like that except to say that at Mainline BA there are a lot of commuters that volutarily bid for Back-2-BAck. That said mainline have a lot of 3 and 4 man crew trips where this kind of flying isn't quite so bad.
Back-2-back bullet JFKs though, IMHO is asking for trouble, wonder what the pax coming back from JFK would think of it if they knew their pilots were on their 3rd sector in 5 days, probably feeling less than 100%
As I said originally, expect a high turn-over of pilots there, your body can only take that kind of punishment for so long!