FYI, I found an easier way to do this - which is good since nobody seems to know the answer to the question I was asking.
Using Powershell you can apply an existing guest customization specification to an existing VM. The code is along these lines
$myspec = Get-OSCustomizationSpec -name 'thisismycustomizationspecification'
$VM_NAME = Get-VM -Name 'thisismyvmname'
$VM_NAME | set-vm -OSCustomizationSpec $myspec -Confirm:$false