The IotaWatt FW is full opensource and is easy to modify and compile (PlatformIO recommended)
By default the IotaWatt monitoring data is not printed to serial, the serial port is used for debugging. However, since the ESP8266 has got two serial ports the 2nd serial port could be setup to output monitoring data. Currently the easiest way to intercept the IotaWatt data is vi posting an an Emoncms sever (this could be local e.g. emnonPi / docker / ubuntu). MQTT support is planned in the future. @overeasy will be able to advise further.