Here is an example from CentOS and a 7-Mode VSIM mounting with NFS only (and a loopback to the lun object after). It is possible, seen in the example below, and I think it is pretty cool. NFS alone isn't handling it, but the loopback of the block device seen only over NFS without using iSCSI, FC or FCoE. The application of this would be questionable if someone had iSCSI licensed. I wouldn't recommend it, but some may have a use for this. The first example is an iSCSI lun where file1.txt and file2.txt are written, then I cloned the volume to export the clone over NFS only then the loopback mount.
Create and mount a LUN iSCSI from Linux
vsim-7m-1> vol create test aggr2 10g
vsim-7m-1> iscsi start
vsim-7m-1>lun create -s 1g -t linux /vol/test/testlun.lun
[root@localhost~]# /etc/init.d/iscsid start
[root@localhost~]# iscsiadm -m discovery -t sendtargets -p 192.168.150.210
vsim-7m-1>igroup create -i -t linux linux iqn.1994-05.com.redhat:a3dd625ebee5
vsim-7m-1> lun map /vol/test/testlun.lun linux
vsim-7m-1>lun show
/vol/test/testlun.lun 1g (1073741824) (r/w, online, mapped)
vsim-7m-1> igroup show -v
linux (iSCSI):
OS Type: linux
Member:iqn.1994-05.com.redhat:a3dd625ebee5 (logged in on: e0a)
[root@localhost~]# tail -f /var/log/messages
Jun 24
17:39:26 localhost kernel: scsi 4:0:0:0: Direct-Access NETAPP
LUN 810a PQ: 0 ANSI:5
Jun 24
17:39:26 localhost kernel: sd 4:0:0:0: Attached scsi generic sg2 type 0
Jun 24
17:39:26 localhost kernel: sd 4:0:0:0: [sdb]
2097152 512-byte logical blocks: (1.07 GB/1.00 GiB)
[root@localhost~]# fdisk /dev/sdb
Command (mfor help): c
DOS Compatibility flag is not set
Command (mfor help): u
Changing display/entry units to sectors
Command (mfor help): n
primary partition p
Partitionnumber (1-4): 1
[root@localhost~]# mkdir /mnt/iscsi
[root@localhost~]# mkfs /dev/sdb ; mount /dev/sdb /mnt/iscsi
[root@localhost~]# cd /mnt/iscsi
[root@localhost iscsi]# ls -l
total 16
drwx------ 2 root root 16384 Jun 24 17:50 lost+found
[root@localhostiscsi]# > file1.txt
[root@localhostiscsi]# > file2.txt
[root@localhostiscsi]# ls -l
total 16
-rw-r--r-- 1 root root 0 Jun 24 17:51 file1.txt
-rw-r--r-- 1 root root 0 Jun 24 17:51 file2.txt
drwx------ 2 root root 16384 Jun 24 17:50 lost+found
Clone the iSCSI LUN volume and export nfs for loopback mount (no lun mapping)
vsim-7m-1> vol clone create test_clone -b test
vsim-7m-1> lun show
/vol/test/testlun.lun 1g (1073741824) (r/w, online, mapped)
/vol/test_clone/testlun.lun 1g (1073741824) (r/w, offline)
vsim-7m-1> lun online /vol/test_clone/testlun.lun
vsim-7m-1> lun show
/vol/test/testlun.lun 1g (1073741824) (r/w, online, mapped)
/vol/test_clone/testlun.lun 1g (1073741824) (r/w, online)
vsim-7m-1> lun share /vol/test_clone/testlun.lun all
vsim-7m-1> exportfs -i -o anon=0 /vol/test_clone
vsim-7m-1> exportfs
/vol/test_clone -sec=sys,rw,anon=0
[root@localhost~]# mount 192.168.150.210:/vol/test_clone /mnt/nfs
[root@localhost~]# ls -l /mnt/nfs
total 17492
-rw------- 1 root root 1073741824 Jun 24 17:55 testlun.lun
[root@localhost~]# losetup /dev/loop0 /mnt/nfs/testlun.lun
[root@localhost~]# mkdir /mnt/loopback
[root@localhost~]# mount /dev/loop0 /mnt/loopback
[root@localhost ~]# ls -l /mnt/loopback/
total 16
-rw-r--r-- 1 root root 0 Jun 24 17:51 file1.txt
-rw-r--r-- 1 root root 0 Jun 24 17:51 file2.txt
drwx------ 2 root root 16384 Jun 24 17:50 lost+found