This is what I would do. Get a smart supported “dry contacts” relay. Hook the dry contacts “normally closed” in series with your old manual humidstat. Turn the humidstat to the lowest setting. You need 110v to operate the relay but I am pretty sure it’s near by since the humidifier is using 110v. Don’t worry about the 24vac. It’s for a solenoid that’s control by your old humidstat. By hooking it in series and using the normally close contacts if the relay failed in closed positon. You will still be able to use the old humidstat.
Edit: an Aeon micro is not going to work.