So the only reason why you are taking this approach is to understand the usage and charge customers accordingly, is that right ?
Why do you need to backup if they are stateless? Do you use vCOPS ? I have a idea, I will have to confirm this - someway group all the customers VM in a folder and ask vCOPS to show you the result like total usage. This may not work for you if you don't use vCOPS Image may be NSFW.
Clik here to view.
I will think further and let you know about it.
