As long as you have admin access to both vCD and vSphere, you can relate objects between the two. Alan Renouf has a excellent explanation here: http://www.virtu-al.net/2013/03/08/relating-vcloud-director-to-vcenter-in-powercli/
You are correct in your assessment that the Get-CIVM cmdlet does not have used disk space. The vCloud API does not have it either, for that matter.
In regards to your code, check your foreach statement. You have foreach ($_.name), then the code block. Try removing the ($_.name)
