I've read a long time ago that some sound engineers would sometimes put dummies in a large portion of a concert hall in order to get the right sound quality, as the hall had better suited reverberation for recording than in an empty concert hall.

it would be interesting to compare a room calibration with and without people in the seats...
has anyone done it? with what results?

See: http://www.blu-ray.com/community/gallery.php?member=Gelli
or: Axiom Gallery