When I'm back home, I will check whether I can find some documentation about best practices with multi-site configurations which may help answering your question. You may also want to take a look at the documentation provided with the system (the latest version is also available on HPs web site). Anyway it looks like the VAR was just thinking of VMware storage heartbeats and not about the storage itself!?
Btw. just out of curiosity, with the "stretched metro cluster" you mentioned, how are the sites connected to each other and what's the distance between them? In case of Ethernet, what's the roundtrip time for pings?
André