Looks like if you have zoned heating, then you can just use the Nests like normal. Is that the case? If not, they also make a remote temperature sensor for the Nest. Then you still have one controller, and one remote temp sensor. The controller asks the sensor what it is reading, and can decide how to react - heat that other room to the target temperature, heat the house to an average, etc. It looks pretty slick - It can target different rooms as priority according to time of day - living room in the evening set to 74, bedroom at night set to 68, etc.
All of that being said, you could still hijack the signals coming out of the device. It runs I believe at 24V AC and essentially closes a relay to request heat and another to request fan. So build a board behind the Nest to read this, and rebroadcast into your ecosystem via MQTT / ESP8266.
Then you build the new receiver controller board which compares the heat requests from all the different devices and it chooses priority when to run the boiler. It gets complicated pretty quickly since you don’t know the target temperature or ambient temperature for each device without the API (discontinued in 2019); and even WITH the API, it requires internet access to get the data.
But the Temperature sensor is only $39 and works out of the box…