You should - at least in theory - be able to use the temperature sensor input of the emonPi, but there would be a lot of development work and you’d need to make serious changes to the code inside the “emon” front end part. And then find the best way to present the data in emonCMS.
The temperature sensors are One-wire devices, and there is the Maxim DS2413 with two I/O pins (so it would do 2 thermostats). Quickly looking at the data sheet, it appears each will accept a switch or a voltage input, and be able to be polled and report on the same bus as the temperature sensors.
Adafruit do one on a breakout board: https://cdn-learn.adafruit.com/downloads/pdf/adafruit-1-wire-gpio-breakout-ds2413.pdf.