On our Hunter controller it was set globally; The intent was to allow a well system to replenish itself between zone runs. So I would think that a global setting would be fine for a delay between zones from that regard.
That said, if you make it per program then I can change it on each program. The only program where it becomes an issue is the "Full-program" / "Long-run" type programs that really tax our well. The shorter programs to water the flowerbeds and whatnot probably don't need the delay so it may actually be better to offer it per program, and gives you extra value-add over the Hunter controllers.
So, maybe an option under "Time unit" on each program page called "Zone delay" or something?