My old server was a centos 5.9 host and that version of s3fs wouldn't allow you to set the uid/gid for the directory you're mounting to.
And what I've found is that backing up to S3 with these latest versions works GREAT!! I am able to create tapes on my S3 mounted backup directory. And the backups complete without any complaint from the bacula system.
However the problem comes in restoring files. Because what I've found is that the restores will fail even if you are restoring just one tiny file. I tried restoring a resolv.conf file.
What happens is that the s3fs directory disconnects from s3 and bacula gives a 'waiting for mount request' that seems to go on forever.