Even if they would have an interline agreement, the ZED are pretty worthless as you are usually at the very bottom of the SBY list behind all the airline's employees + families. As many airlines nowadays run at fairly high load factors (especially in the USA), unless you are single with lots of flexibility and spare time/vacation, forget the ZED's, not worth it. Easy staff travel's strong point is the fact that you get firm seats, so are treated like any revenue passenger. Firm seats run even cheaper then BA SBY (tax included). All in all, not bad in my opinion.