Hi,
Is there a command to show what extra contribs / rpms have been added to an SME server?
I don't believe so, no.
How would one find out what had to be reinstalled prior to restoring a backup on a customised installation?
If you asking about restoring a backup to the same version of SME Server that you created the backup, that's straightforward: Compare installed packages to the package set on the install CD. I.e.
# rpm -qa | sort > installed_pckgs.txt
# mount /mnt/cdrom
# ls /mnt/cdrom/CentOS/RPMS | sort > default_pckgs.txt
(/mnt/cdrom/e-smith/RPMS for earlier versions)
Note that the package set listed in default_pckgs.txt doesn't represent all packages that are installed by default... some packages are probably provided with the CD but not installed (see /mnt/cdrom/CentOS/base/comps.xml). Also, you can't just diff the two files to find the difference... the default_pckgs.txt list will contain "duplicates" of some packages for various architectures, and it will contain file extensions that aren't provided in outout of 'rpm -qa'.
If you are asking about restoring a backup from one version to another, thats a different, but similar, story.
HTH,
Paul