Hey Darrell, great investigation and write up! More fodder for fun mods...
Looking at your numbers (13.2V, 23-27 ohms, and 30% duty cycle), I see the ESU and Lionel smoke unit heaters are running around 2W to 2.25W, which is pretty consistent with where the MTH ones end up - usually around 2.25 to 2.5 W. So the decoder seems ok with that power output at a current around 0.6A. Tempting to try it on an MTH unit (at a lower duty cycle, to get the same average power), but I'm guessing it might choke on the ~8 ohm element, as that would be in the neighborhood of 1.5A while powered on.
Nice hack with the thermistor on the heater board! It makes a lot of sense that the ESU smoke unit has that built in. Were you able to tell while experimenting if the temp sensor is actually in the control loop for the heater? For instance, does the heater run near 100% duty cycle while first heating up, and then drop to the 30% steady state once hot? That would suggest the thermistor is actually used to control the heating, which makes a lot of sense. If not that, maybe it's just there for overtemp protection or something. The wording in the manual is a little vague.
I'm a little surprised you had to dig into the sound file like you did. That option is not available on the Loksound Selects, and they clearly are supposed to support the smoke unit as well. Here's the text from the Loksound Select supplement:
The decoder reads the data from the temperature sensor and controls the heater based on the desired settings, so the smoke unit could never burn out. Please note that the settings for the smoke unit need to be included in the sound project file. While you could easily change these settings on LokSound L V4.0 decoders, the settings in LokSound Select L decoders could not be altered by yourself. ESU will do the conversion of the LokSound Select files to be compatible with the Smoke units.
I wonder if that particular sound file just wasn't updated recently or something?