Well, the tuner matches what's in front of it to what is behind it.  with 
the new technology and moisture proof boxes, its worth considering, since 
having the tuner at the base of the antenna should transfer the most power 
to the radiator, instead of the radiator and coax.  I doubt if this would 
work for a balanced fed antenna.
The QRP folks here may have better info on this idea.
I don't think that their are high power remote tuners, but 100 watts can be 
done.