In my old diesel Rabbit I installed a switch like you suggest to kill the
comprssor when going up grades and such.  In that car you had to slide the
selection lever over several spaces to turn it off there was no off on the
fan.  Since I usually just wanted to turn it off to get up an on ramp or
something similiar and in the summer still wanted air to move I put the
switch in.  Really simple to do to, just splice into the single wire going
to the clutch on the compressor.  In the Vanagon, due to the long lines to
the compressor, I might use a relay in the engine bay to do the same thing.

Dave K.