Main problem I see is a build-up of algae and mold and the like. Last thing I'd want to do would be spreading that around with my humidifier.
I could probably rig something with a UV light, but that would be expensive and I'm not sure it would be all that effective. Filtering would be another way of going about it, but again, seems like over kill to me to rig up a big activated carbon filter.
The humidifier tank holds 3 gallons, but it has a good handle, so that would be all that much of an issue. I'd need some sort of pump to fill the tank, though.