Routing through JFK is possible on the non-LHR routing with the same Matrix parameters.
The fare rules allow three free stopovers. I have not actually tried to generate those itineraries.
Amazing! Trying to find an nyc-cmb-nyc r/t I can stick this in the middle of.
FYI, Re: jfk, I can't get the stopovers to work anywhere but Vegas without bumping the price quite a bit.