その後

作ったFFS領域を/exportにマウントしてからsmb4.confをZFS仕様からUFS仕様に変えたところ、無事DFSが動作。
せっかくのZFSが活かせないのが残念だが、とりあえず利便性向上。

# rcsdiff -r1.2 -r1.3 smb4.conf
===================================================================
RCS file: RCS/smb4.conf,v
retrieving revision 1.2
retrieving revision 1.3
diff -r1.2 -r1.3
1c1
< # $Id: smb4.conf,v 1.2 2014/07/08 10:29:46 root Exp $
---
> # $Id: smb4.conf,v 1.3 2015/02/07 03:43:59 root Exp $
11c12
<       server services = rpc,wrepl,ldap,cldap,kdc,drepl,winbind,ntp_signd,kcc,dnsupdate,dns,smb
---
>       server services = rpc,wrepl,ldap,cldap,kdc,drepl,winbind,ntp_signd,kcc,dnsupdate,dns,s3fs
12a14,16
> #     vfs objects = zfsacl dfs_samba4 acl_xattr
>       vfs objects = dfs_samba4 acl_xattr
> posix:eadb = /var/db/samba4/eadb.tdb
15c19,20
<       path = /var/db/samba4/sysvol/********/scripts
---
> #     path = /var/db/samba4/sysvol/********/scripts
>       path = /export/scripts
19c24,25
<       path = /var/db/samba4/sysvol
---
> #     path = /var/db/samba4/sysvol
>       path = /export/sysvol
24a31
>       guest ok = yes

(一部隠蔽してます)