Hi,
You won't be able to share a RDM between cells without some kind of cluster filesystem at the OS level as vCD cells in a cluster are all active, probablly more hassle to implement then just implementing a VM based NFS server, and almost definately not supported (but you'd have to check!)
So that sort of leads into the second questions, which is yes you could implement a NFS server in say Redhat, and serve the share that way, maybe protecitng it with FT/HA/Stretch cluster/vMSC depending on your SLA's
The impact of the NFS share going offline is not catastrophic to the cell. During the period it's unavailible you won't be able to upload OVF templates, or provision servers across PVDC;s in different vCenters, but one it become availible again it should reconnect automatically just as any NFS share in RHEL would.
HTH
Steve