I've got a bypass humidifier mounted on my gas furnace, and an ecobee 3 thermostat driving both furnace and humidifier. What I like about having the ecobee drive the humidifier, is that it knows the outside temperature and so is able to adjust the target humidity level to avoid condensation on my windows.
The problem is that here in Colorado, the air is so amazingly dry that even with the humidifier running 100% of the time that the furnace is running, it can't get the humidity over about 28% - even when target humidity (based on outdoor temperature) is over 40%.
The ecobee has a setting for "type of humidifier" where the options are "steam" or "evaporative". Right now it's set to "evaporative" but according to their tech support, setting it to "steam" would allow the humidifier to run while the fan is running, even if the furnace isn't (and I have my system set to run the fan 75% of the time, to try to keep the different parts of my house at the same temperature).
Does running the humidifier when the furnace isn't heating have any risks? Does the humidifier somehow depend on the fact that the air flow it's dumping into is recently heated?