From what I understand, QNAP does not currently support DRBD. So, I'm not sure what all is available for HA on QNAP, natively.
You can use Real-Time Remote Replication for replicating between two QNAP devices. However, this won't work on iSCSI LUNs without taking the LUN offline. Even then, it seems to be a bit convoluted to get working.
I'm Elliot. I'm your friend.