Apologies, this was the reference I should have included:-

Features of Writable Volumes

"You can understand how Writable Volumes can be used with VMware Dynamic Environment Manager, behavior of Writable Volumes in a non-persistent virtual desktop, how Writable Volumes are used in a shared datastore, and how to exclude specific locations of Writable Volumes from being overwritten."

There is no mention of persistent machines as they are indeed unsupported.