You really want to be able to see a iscsi target, import it somehow into lvm's database then mount it read-only on your backupserver I gather?

Does your backup server have the same OS or does it support all the fancyness of LVM2 with ext3 filesystems? Our backup servers wouldn't - they are all Solaris.

Just share out the target and make sure you can see it with your disk formatting tools on the backup host and you can kind of follow this:

http://www.linuxjournal.com/article/8874

Let me know if that was enough to get you started - I am thinking of quite a few pitfalls and I want to see if you encounter them using lvm