There isn't such option in MapServer. I suppose that for getting such option a new geometry transformation algorithm should be implemented http://www.mapserver.org/mapfile/geomtransform.html. First thing to do is to define how to measure the rotation angle. Perhaps it could be the direction of the longest side of a minimum rotated rectangle that encloses the whole geometry.
Only option that comes into my mind is to use some external tool for computing the rotation angle and write it into a new attribute in your polygon data. Then you could use the attribute based rotation as described in http://www.mapserver.org/mapfile/label.html.
Alternatively if you could generate the center lines of the polygons, or perhaps better the center lines of the minimum bounding rectangles of the polygons, into a new line vector data set you could add a new line layer just for labeling with ANGLE AUTO. If lines have no attached styles only the labels will appear on the map. There are utilities for computing the centre lines and sometimes the process is called skeletonizing.