Oh, wait a minute sgw, "Humidifier motor"? If this the rotating wheel style humidifier then heed paul's advice. I was thinking that you were just looking to power the water line solenoid.
I recently installed a motorized damper on my combustion air intake vent and knew that the control/thermostat transformer wasn't rated for that, so I tied the damper into the humidifier transformer. To prevent over humidification, I simply turn off the water supply at the saddle tap until later in the season when I need humidification with every heat cycle.