Thank you for the reply, nayr!
What you described will work well for the fully automated case. But what about the ‘manually’ operated case I’d described?
I have a physical toggle switch out in the garden that I flip to energize that ‘master’ valve. I turn it off when I’m done hand watering.
When that manual switch is activated, it’d apply 24VAC to the wire feeding the solenoid. This same line would also be connected to the ‘master’ port on the OpenSprinkler. However, I’m not sure if the circuitry would tolerate that? There’s the MOV and the triac on each output…I could also look to see if the triac would support this sort of use.