I found a brass hex head in the center of the circular plate the manual called the 'crankshaft box cover' in the diagram with the manual. This brass hex head nut has what appears to be a circular piece sintered brass about 1/2 - 5/8" in diameter.According to the really crappy diagram in the manual, there is what looks like a flapper valve right behind this that is labeled 'valves'. This particular structure isn't even labeled in the manual. It would appear that this brass hex head with it's sintered metal center is an air inlet. If so, I suppose it could be threaded out and replaced with an adapter to connect to a more aggressive filtration device. That said, I'm reluctant to take a wrench to this thing without knowing for sure if it can be safely removed.
I do love this little compressor, but it's manual is utterly, completely and totally worthless. The same is true for the SuperCharder and TurboCharger compressors. Omega or whoever REALLY needs to correct that.