Well, the auxiliary output on the EcoBee 3 can be used for a ventilator, a humidifier, or a dehumidifier. However it’s only rated for like 24 votes and relatively low amperage . It will work fine for a humidifier that is built on to your furnace, clicking the water relay on and off. You need to tap 24 V off of your furnace run it through the relay and then to the humidifier than the other side of that relay goes to ground. It works great and it’s how mine is currently set up. In fact I was having trouble and I called them and their tech support walked me through doing it and explaining that is just a dry contact.
With that said if you had a external humidifier that you want to click on and off based on the EcoBee telling it when to do it you can use a RIB relay rated for 24 V on the pilot side and 120 on the voltage side (RIBU1C ?) to turn on and off a full size machine.
@storageanarchy - can you see the value of the aux output through your DTH? I.e. If someone set it up as a humidifier can you see it turn on and off and maybe they can just use that to turn a standard zwave/zigbee outlet on and off?