It would be nice to be able to locally generate a fully customized sysprep file and then use it in a non-persistent OS customization spec. This would allow per-vm customizations to be injected into the unattend file via script without having to save each unique customization spec in Virtual Center.
I think it might. I created a new customization spec in the GUI and imported an unattend.xml file. I then went digging through the spec using the Get-OSCustomizationSpec cmdlet and in the object I found $spec.ExtensionData.Spec.Identity contains the full copy of the unattend.xml.
I'm going to try working on that further, but it may be possible to create a temporary customization spec (or clone a sysprep based one) and then update that value with the XML you need.