OK, I have discovered that it is necessary to run the phantom zone in a schedule (not a quick run) in Rachio before all the attributes of the zone are available and controllable. The fact that there really isn’t a valve attached to the zone in Rachio is irrelevant.
So I have achieved success in having Rachio control an Orbit garden hose timer as if it were one of its zones, which relieves me of using the Orbit app and it coordinates all my watering zones, obeying weather skips, etc.
Again, the glue here is home automation that can talk to both APIs. Same could be achieved with IFTTT if and when Orbit supports being an action in IFTTT.