So I did use some time to implement a gw with local sensors.
And to be honest I gave up.. The existing code is awesome and don't have that much room to implement it. I did make some progress, but all that would be undone with an update.
So compared to the time it took me to wire up an Arduino Pro Mini with humidity and temperature sensor(20min) and the cost of it. My life is to short:)

/M

That lib only supports HW SPI(from what I can see) and I would need it for Pro Mini's. But tried it on a mega, almost same issue.
Did some more testing with same result. I checked the datasheets , and from what I can see, it is depending on a stabel reference voltage.
Was driving my pro mini with 5v and then regulating it to 3.3v for the MAX31855. So dataline was 5v and max31855 was 3.3. Even if this is within spec the macx31855 board I got was as cheap one, so it did not filter the voltage/data input for the chip.(Looks like the Adafruit board does?) Moving the regulator in front of it all, driving everything with 3.3v it's as stabel as could be expected.

Green is a DS18B20, Blue a DHT21 and the black is the ThermoCoupler.