Well I did a little research and the most common material that AC filters are made of is FIBERGLASS. I don't think that would be too cool for plants. The blue type is coated with some sort of protectant to keep the FBG from breaking down and dispersing through the AC/heating system.
I will probably be getting some...ta da....EPIWEB in the near future....
Rockwool (AKA fiberglass) is a well established hydroponic medium, but not with the dust-collecting coatings used on filter media or the binders used on insulation.