+1. Ray, take my money, now! I just installed OSPi this past week–very easy install, no hitches–and was thinking about how thermostats use 24VAC for switching, just like a lot of sprinkler systems do (mine included). Of course, sprinklers use solenoids, and HVAC switching is for the control side of a relay, but both are just driving coils in the final analysis.
A system that didn’t need a Pi, like the ESP8266 you mentioned, would be even better cost-wise, as long as it could provide similar functionality. I know that the Pi is a bit of overkill for OpenSprinkler, but it’s at such a good price point that the overkill doesn’t mean overpriced.
Can’t wait to see this come to fruition!! Loving my OSPi so far!!