Which part of the GUI are you referring to? I have set up a shared LVM storage type using iSCSI LUN without any issue. I don't see the LUN under the node's Disk section and I don't know why. But I can still add a LVM storage under Datacenter > Storage > Add (LVM). In the wizard, choose "Existing volume groups" for Base Storage, and the Volume group drop-down should show all available volume groups on the node.
The node's disk section seems to show local disks and FC LUNs only.