If you are specifically looking for a wordpress plugin, there are others that are more sophisticated than the DreamObjects one and should work with larger sites. For example, this one is quite popular:
Otherwise, a script could be written in bash to make the needed backups and upload it to DreamObjects, and ran by cron. You could do a mysqldump or a tar and pipe it to "s3cmd put" to backup as well. I don't have any example scripts to point to but we can look into adding some examples into our knowledge center.