Something like this would do (you may want to remove some of the excludes like logs for example).
cd /
tar -czf /backups/linux1.tar.gz ./ --exclude="backups" --exclude="tmp" --exclude="var/tmp" --exclude="var/log" --exclude="var/cache" --exclude="proc" --exclude="mnt" --exclude="media" --exclude="sys"
Tar's --one-file-system argument could also prove useful in your situation. Check the man page.
You could then write a simple for loop to backup all your mounted Linux installs.