For the record, I'm also interested in this. I have three of my zones in the back yard that I'd like to add a master valve for, so the main line to the back yard isn't always pressurized (there are some problems with that line...).
Until that gets implemented, though, could I somehow tie the master valve to all three (in my case) zone terminals? With diodes or something to make a logical "OR" function? (The goal being to connect power to the master valve from any of the three zone terminals, if any of the three are powered up)