I got the Radio Thermostat CT50 thermostats in and they were very easy to set up on a private-not-internet-connected network, and then access via their well documented RESTful HTTP API. There's no security at all, but that doesn't matter because of where they are. I'm not sure how to set up security, but maybe it's possible.
Getting them working with the rest of the system was very easy. I didn't use the perl module I mentioned earlier because I decided to go down one level and use a REST client perl module in order to access more API calls than the dedicated module exposed. I guess I should really extend and propose changes to the dedicated module - maybe soon. Anyway, the main thing that was missing for me was the function to set the date/time on the thermostat - something that drove me batty with the Insteon thermostats. It was very easy to do on the CT50 -- but really you only set the day of the week, the hour, and the minute. This means that they can be close to a minute off after you set them (if you set them at say, 09:59 at 09:59:59, then the real time changes to 10:00:00 one second later, the thermostat will be 59 seconds behind. Whatever)
Like most thermostats, they have a "heat" mode and a "cool" mode - like some, they also have an "auto" mode that will choose either heating or cooling as needed. Right now I'm simply in "cool" mode because it's kind of hot outside here. Adapting my simple user interface to include the complexity of the thermostat is kind of tricky. I added a command for "cooler" that takes the current temperature, subtracts 2 (with a floor) and then sets a temporary target on the thermostat to that new temperature -- the otherway with "warmer" - but it's not always obvious whether it will be necessary to switch the mode or not to comply with the request. I guess I'll figure that out later, and maybe just wind up in "auto" all the time.
The thermostats are programmable on a weekday basis and I don't plan to create a user interface to do that on my phone - I'll just use the existing touch screen interface for that part.
RTCOA bucks my initial gloomy prediction about home automation - you *can* offer a device with user-friendly vendor cloud-based management *and* have it easily function on a private-only network and hook into a custom user-controlled system. Totally crushing on them right now.