Those scripts are meant to interface with a Macintosh running Mac OS X Server, which has Software Update Server as a feature of the OS. If you upgrade your G5 to Mac OS X Server, it should be able to host software updates for the rest of your machines to use via Software Update, which you would configure using that script. A cheaper way of doing this is to just use it as a file dump with copies of all the Combo Updates from Apple Support in a shared folder and just run those locally as a copy of the folder on your G5, however what you describe requires your G5 to be upgraded to Mac OS X Server.