I am needing the ability to turn the master valve (running a rust treatment injection pump) on or off uniquely in each zone as some zones are hardscaped (needs to be on) and some are not or my fish pond auto fill or garden vegetable irrigation (needs to be off). This is a deal breaker for me to be able or not to use a Rachio at my home. Runs great at our cottage as no rust treatment system there…
It seems that currently, the master valve is selectable/deselctable if one exists on your system.
I would recommend installing a second controller that could run your rust treatment on its appropriate zones by itself. It’s only a few dollars more but will give you that flexibility that you desire.
I run fertilizer injection systems on drip with this method as I don’t want the same plant fertilizer on my lawn and vice versa.