I didn't find a command line function that does the same operation as the GUI button "Reset crop to maximal possible area".
The pano_modify --crop=AUTO command does the same as the "Fit Crop to Images" button in the stitching tab (expert mode) but that's a different function which creates an optimal crop whereas the GL view button resets all crop to zero.
Right now my solution is this command :
sed -i "s/\(p .*w.* h.* v.* k.*\) S.*\( n.*\)/\1\2/g" "file.pto"
but apart from being very ugly it's probably not very future-proof for upcoming versions of hugin.
Is there no built-in command for that?