[NFS] re-exporting NFS-mounted dir over NFS

["Martin Schuster (IFKL IT OS DSM CD)" <Martin.Schuster1-d0qZbvYSIPpWk0Htik3J/w@public.gmane.org>]

I am in the process of setting up an "NFS-proxy", a machine which
mounts directories from an NetApp-filer using NFS3, and should re-export
them using NFS4 (with Kerberos).

But apparently nfsd doesn't support re-exporting NFS-mounted dirs:
# mount | grep home
netapp.example.com:/vol/home/schumar on /srv/nfs4/home/schumar type nfs
# cat /etc/exports
/srv/nfs4/home/schumar  gss/krb5(rw,sync,fsid=0,secure,no_subtree_check)
# exportfs -r
exportfs: Warning: /srv/nfs4/home/schumar does not support NFS export.

I traced this back to linux-2.6.25.1/fs/nfsd/export.c, where it says, starting
in line 386:
        if (!inode->i_sb->s_export_op ||
            !inode->i_sb->s_export_op->fh_to_dentry) {
                dprintk("exp_export: export of invalid fs type.\n");
                return -EINVAL;
        }

(and a quick look in fs/nfs/super.c confirmed that the nfs-client never
sets an export_op)

Is there a technical reason for this (i.e. is it simply theoretically
impossible to re-export an NFS-mount), or is my use-case so strange that
nobody has ever needed this until now, and thus it just wasn't coded?
Or am I just stupid/blind? (wouldn't be the first time :)

Thanks in advance,
-- 
Infineon Technologies IT-Services GmbH   Martin.Schuster1-d0qZbvYSIPpWk0Htik3J/w@public.gmane.org
Lakeside B05, 9020 Klagenfurt, Austria   Martin Schuster
         FB: LG Klagenfurt, FN 246787y   +43 5 1777 3517