Is the following scenario somehow possible:

(Site1)
2 x Open-E DSS in active/passive configuration for iSCSI
Hypervisors (Xenserver) connected to High Available target

VPN over WAN (100 mbit / ~ 10 seconds latency)

(Site2)
1 x Open-E DSS
Hypervisors (Xenserver) standby

Setup a replication between Site1 and Site2 (in fact, a second replication). In case a disaster happens at Site1, or major power/network/cooling outage, boot the hypervisors at Site2 and log-on to the replicated targets at Site2 and restart VMs here.

Citrix Xenserver supports Site Recovery in version 6, which is built upon some sort of replication between iSCSI storage (no matter what storage device) but I cannot see how this fits in with Open-E. Maybe someone can shed a light on this issue. I would really like to also keep the active/passive failover at Site1 to avoid switchover on 'just' storage failure.