You cannot "backup" the underlying "physical" iSCSI LUN (which are actually vSAN components etc.) served by the vSAN iSCSI Target. So you have to look at it from the perspective of the VM's or physical hosts you are trying to protect. From their perspective, iSCSI disks are actual LUNs and not virtual disks (VMDK's) in the classical sense. In your head, treat a vSAN target generated LUN, presented to a physical or virtual host, as a LUN from a regular "classic" SAN.
So to answer your question: use agents inside the hosts that are connecting to those iSCSI LUN's (I know you did not want to hear that, but it is what it is.)
