i dont know about xfs nor xfsdump.
i can say that dump is cool tool. it can backup complete of partition. also u can define dump level. so it backups only differences between previous lower level dump. as i remeber it has 9 levels. u can write a simple script and add it as cron job. so u can backup your patitions in different levels for different days of week. so u will have a backup system backwarded for everyday of last 7 days.
tar is as cool tool as dump
i think u can add files which are modifed or created in recent (3) days to tar file like this:
find / -mtime -3 -type f | xargs tar rf /path/to/file.tar
but im not sure if it is a good idea. becuse it will probably try to add file.tar in same file.tar
find / -mtime -3 -type f |grep -v "/path/to"| xargs tar rf /path/to/file.tar
now it seems that it is a good idea
i love linux, because u have lots of choices
edit : u dont have to add newer files in that file.tar, it is better idea to backup newer files in another tar file.