Hello cfor,
" I also know from testing a disk can be added via vSphere API's to a multiple vCloud VM and it works - however as I understand this is not a supported configuration."
mean to say we have to connect with vCenter and attach disk to both the machines?
This again requires access to vSphere resource, which is against to idealogy of Cloud where resources are abstracted from end-user to have greater security.
If possible a System Administrator from cloud provider can do this but an end user sitting on other edge of cloud can not on his own.
please correct if misunderstood.