Ok, but the replacing would occur for non config files like firefox, firefox-bin, python files in /usr/local/bin that would not need copying in the case of a load-to-system use. Though those files could be stored in, say, /usr/local/share/firefox/ as firefox.orig or firefox-bin.orig and then be copied to the system. But that would be copying them in the case of a load-to-system instance where copying would not be needed, only in the tested case of mounting/symlinking. Either way would not be wrong I think, though.