Note that this doesn’t always work with smart thermostats. From first hand experience, there are some models out there that will generally work fine but will not complete booting up if one of the outputs is detected as closed.

Yeah, in that case you can get an old thermostat that disconnects easily and wire it somewhere out of the way.

(Though replacing a smart thermostat with a mercury switch one is a five minute job if you're comfortable with it.)