The BATF considers anything that can be used to suppress the report of a gun to be silencer. I remember reading an article, though I can't find it now, about someone who had made an airgun silencer and sold it to someone. It was designed specifically as an airgun silencer, and had a felt lining or some material that would burn and disintegrate if used on a firearm. He was arrested and sentence to something like 10-15 years for manufacturing and selling a silencer without the proper permits.
Also have a look at what Mr. Beeman has to say on silencers:
In short, either don't try and silence your airgun, or go through the BATF paperwork as though it was a firearm silencer.