I finally managed to have the temperature sent through serial. The temperature must be sent like this:
“\nserialremotetemp=1500” (the temperature must be between -3000 and 12000 because the value gets divided by 100 in the source code).
At the moment I have a remote Moteino node with an RFM69HW module (which sends the temperature from an MS5611 barometric sensor) and another Moteino + RFM69HW connected to the Rx of the ESP8266 that acts as a gateway and forwards the temperature value to the Thermostat.
The only downside is that the remote temperature is not sent to Thingspeak.com (in case you use it). The source code must be changed for that (I might try …).