I don't disagree that you should use caution with fertilized water dripping from hanging plants, but organic fertilizer is not any different chemically than synthetic fertilizer and organic does not mean harmless. The Osmocote would be safe, even used in hanging plants, as it is slowly released and won't be releasing harmful amounts of fertilizer from the run off. If you are paranoid, simply buy a bit of bagged manure and repot your plants with some and add some worm castings, every 3-4 months. I have kept terrariums with sensitive amphibians and fish, while properly and carefully using fertilizers and it's perfectly safe when done with common sense.