Unless I’m misunderstanding, I don’t think this idea will work.

In most programmable thermostats, the “R” terminal on the thermostat is both the source voltage to power the thermostat and is also the same power to each energized output.

Think of the thermostat as a switch to each output. The thermostat makes decisions about when to switch ON the signal voltage from the furnace power source back to the furnace devices/controls which operate the heating functions (ex. relay, contactor, etc…)

You definitely can use a transformer to step down the 120V power but the 24V thermostat outputs will also be 24V back to the furnace. In order to make this idea work, each thermostat output would individually then need to be transformed back to 120V since the current controls internal to the furnace or controls associated with your heating setup are looking for a 120V signal.