ANR
Foggy,
I am not an audiologist either, but if we just concentrate on what's entering the ear itself, ANR really does cancel the air movements by applying counter waves with the speaker. Given that the audio "signal" (noise) is fairly complex in waveform, this is probably not an easy thing to do. It appears that today, however, many companies have the technology and the knowledge to do this very efficiently.
The rest of the noise that ruins your hearing and gives you a nice headache reaches the ear drums via other routes, like the cranium and on via the soft tissue inside of it. That's where the helmet comes into play.
In response to your "masking" statement, which is true in a sense, I would say that if the airwaves are gone, then the noise is gone and the result is harmless since it produces no movement of the ear drum.
Anyway, a helmet with ANR-cups in it must be the optimal solution.