Add a wire terminal to the Rachio to connect to a fire alarm panel, or an automation system, to signal emergency start up of zone watering. Have some kind of zone watering pattern pre-programmed for this - like rotate through watering every zone for 3 minutes at a time.
This would expand the Iro's emergency-start-on-fire capability beyond what you already have for the Nest, to include a lot of fire panels and almost all automation systems (without you having to research each and every one for compatibility concerns).
I'm specifically suggesting a wired terminal, not a wireless capability. Not only do I think this is lower cost to implement in the Rachio, it also provides reliability for signaling a fire related event.
Just a suggestion (that I would use: my fire panel signals automation system of fire mode; I'd run a cable from automation system to Iro; relay contact closure in automation system signals to Iro to shift to 'emergency mode;' (you'd have to decide if the product feature should respect 32 degrees or colder or ignore it and water anyway ).
An example is HVAC units. Most have a terminal these days for connection to a fire panel or automation system, that upon dry contact closure, shuts down everything associated with the unit (don't spread smoke throughout hose. My suggestion is just like this, except it's for the sprinkler controller now too.