The way to achieve this would be to create a "staging" plan which has the Staging websites resource and provisions to a server group containing only the server(s) you want to use for staging. Then create a "live" plan which only provisions to the non-staging servers and only contains the Websites resource and not the Staging websites resource. Subscribe your customer to both plans. When they create a staging website under the staging plan it will be placed on the staging websites. When they "push live" to the live website it will be placed on the live web server group.
