How do I svMotion a single disk in PowerCLI 5.5?

Trying Set-HardDisk gives the following warning:

WARNING: Parameter 'Datastore' is obsolete. 'Datastore' parameter is obsolete. To move hard disk to another datastore and change it's storage format, use 'Move-HardDisk' cmdlet instead. 

Set-HardDisk fails anyway with a message that the disk is locked (hence I guess you can't use it for an online migration).

Move-HardDisk only seems to be able to move disks in offline mode too (cold migration), however I'd like to perform a Storage vMotion.

Move-VM can only move all disks of a VM, which is not what I want. (Too many and too big disks on a VM, will never fit on one destination LUN)

In essence, I'd like to script what you can do with the "advanced" dialog for migrating disks in vCenter Server. But how?