Your assessment sounds correct to me.
Conceptually this would be more like a discovery/registration rather than the standard provisioning workflows. I've seen/heard some more elaborate professional services projects where this is done... Basically the request is for a set of resources and the provision workflow detects they're in a pool and register them in vCAC (i.e. assigns the machine to the users ownership, group, and template, etc.) This would be a similar type of feature if integrating to VMware view as a provider of desktop as a service.
The "workaround" sounds right, but it somewhat defeats some of the purpose, correct??
If we would design such a feature, I wonder if it would cover your use case by addressing it as some sort of "self service" registration which would simplify what is the "Discovery Wizard" to be front ended by the standard request forms, and resources would come from existing machines as defined in the blueprint. This would probably require some sort of pooling or revised reservation logic to determine what "pools" of machines would be available for registration. If this were in place, the PVS registration/unregistration would layer on top of this and most likely require little to no change...
Not sure if this is the scenario you're attempting to address.