To edit a single image with the filename "foo.svg":
inkscape --verb=EditSelectAll --verb=AlignHorizontalCenter --verb=AlignVerticalCenter --verb=FileSave --verb=FileQuit foo.svg`
To edit all svg images in the current directory:
inkscape --verb=EditSelectAll --verb=AlignHorizontalCenter --verb=AlignVerticalCenter --verb=FileSave --verb=FileClose *.svg
But this second command will open a ton of windows, which is just bad news all around. Here's a better command (for Linux, at least):
for img in $(ls *.svg) ; do inkscape --verb=EditSelectAll --verb=AlignHorizontalCenter --verb=AlignVerticalCenter --verb=FileSave --verb=FileQuit $img ; done