I have a razberry and started on a http driver...the deeper I got into it, the more I realized that zwave protocol doesn't make much sense for an irrigation controller. The protocol will handle it but when you consider the interview process of a zwave device, it becomes more difficult to define the behavior of a device that may or may not have anything connected to any of the binary switches that it would emulate. Now if you wanted to build an irrigation controller using zwave, that makes more sense as you will be defining discreet devices for each valve and you can use sensor polling to emulate flow sensor behavior. I was heading down this route until I realized that smart irrigation controllers were going mainstream.
Very possible for rachio to do this but it will have warts. Much like zwave garage door systems, if you have looked at those, those too don't fit the zwave spec very well and required custom hardware to hide the warts.