This is a great thread.
I have been looking into this for a while and there is another equation (Hargreaves-samani) which is designed to work with a minimum of weather data.
It only requires min and max temperature. Both Penman-Monteith and Hargreaves-samani use latitude as part of the calculation.
WeatherUnderground can be used with Lat-Lon as location data. This is more specific than postal code or city name and can be used world wide.
For the Python Interval program I already have Python code for both equations but if someone wants to write a C / C++ (or any other language) version there is a PDF file containing step by step instructions at:
http://biomet.ucdavis.edu/Evapotranspiration/PMdayXLS/PMdayDoc.pdf
Calibration of the irrigation system is an important part of getting this to work properly since the calculations provide irrigation requirements in terms of inches or mm of applied water (like inches or mm of rain). This also allows for taking precipitation into account.
OpenSprinkler and OS Pi both support a percentage adjustment to the irrigation time as “Water level” so there is a start.
I look forward to seeing how this develops.
Dan