I don't think there is an API to do that, but I may be wrong.
Have you tried to change hard disk name using some other tools, like vSphere Web client or Power CLI? If these tools allow you to change disk name, then we can check what vCenter API they use, and check if this API can be invoked from vRO scripting.
There isn't even a way in any of the UIs to change/update the VMDK name. It would be a manual operation to detach the disk, rename both files (metadata and extent), then reconfigure VM adding the new name back.