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=FileQuit *.svg`