I don't think there is anything to stop you copying DVD's from various regions, it's more down to the Apple DVD player software stopping you 'playing' DVD's from different regions. But it gets this info from the DVD drives firmware, which depending on the model of drive you have, can nearly always be modified to region free. I believe G5 macs use Pioneer 107 drives? in which case, there is defnitely patched firmware available for that. but as i say, I don't think you will need this just to 'backup'
Also, Many DVD 'backup' apps strip out the region code I believe anyway.
Hope that helps some!?