I may not be remembering correctly but.....
It seems like 30 years ago when I was building Transistor Audio Amps, that the Thermistor was placed on the heatsink with the TO3 transistors to limit the current as the transistors (and heatsink) got hotter, thus keeping them from burning up.
So, why doesn't the Nernst lamp have its Barretter mounted right next to the ceramic element?
Wouldn't this be the BEST place to give thermal feedback?
Also, after thinking about it, the thermistor if placed close to the ceramic element would most likely melt!

I may have located some nice small iron wire in hydrogen barretters, but I don't know there specifications. Or for that matter, I don't know the specifications of what we need. (For instance the room temperature resistance/impedance, the temperature at which its resistance/impedance were to start changing and also the max temperature.)
Input?