**UPDATE** Turns out that the custom property below does work for me, in my lab. I had placed it initially in the storage property set of the blueprint, instead of as a blueprint property. So, at least for me, it does work but the solution below may be helpful for other use cases (like a resource action to allow a machine owner to inflate a thin disk).
**UPDATE 2** Sorry that this issue is a moving target, but after looking at this with my customer it seems that the issue is related to Storage DRS in some way. I'll update this post as I learn more.
What they expected (as I did) is that the custom property VirtualMachine.Admin.ThinProvision set in the blueprint with a value of "false" would deploy the machine's VMDKs as thick. Just a side note, if you deploy from template in the vSphere client, you are given the option to select the virtual disk format (i.e. "Same format as source" or thick, thin).
You can "inflate" a thin VMDK by browsing to it in the datastore browser and right clicking. However, it occured to me that this could be used as a work around for my customer using vCO and the vSphere plugin. So, I wrote a simple action that will inflate a VMDK if you provide the vSphere virtual machine object and the uuid of the VMDK - both bits of info are available using the vCAC Extensibility workflows in vCO.