Sound in one ear only?
 

I need to present a sound 100% in only one ear or the other.
I realize it's possible to position a sound source next to
one of the observer's ears, but there is always some sound in the
opposite earphone.

Is it possible for you to write a plugin which will allow me to
control audio "panning" so that I can pan a sound to the
right or left extreme?

How about creating two sound files, one for each stereo channel.