I was thinking of 3 options:  1) what is your switch infrastructure ?  On Cisco MDS you can create read-only zones between host and storage.   2) Part of you Vranger backup script, set volume read-only using diskpart (diskpart.exe /s setvolume.txt   ..where setvolume.txt contains these lines 
Hi,  We have a CX4-120, and are using vmware.  We are using vRanger for vmware backup, and like to start using LAN-Free backup, but for security vRanger needs to connected to the vmware LUNs but as Read only.  Can that be done, and how ?  Regards  Michael Dahl
Read only lun