Since your exporting the storage as iSCSI, the host machine will see it as a raw disk, irrespective of how it's setup on the exported server. So, yes, you can do this.
Sorry for my ambiguity, I meant that mdadm would be on the host machine.
e.g. Using just a 2 node, raid 1 situation Storage 1 -> Disk exported on 192.168.1.10, 192.168.2.10
Storage 2 -> Disk exported on 192.168.1.20, 192.168.2.20
Host -> mdadm multipath md0 = 192.168.1.10, 192.168.2.10 -> mdadm multipath md1 = 192.168.1.20, 192.168.2.20 ---> mdadm raid 1 md2 using md0 md1