Automatically activate a new service on first Authentication

Looking for a simple way to create a service in a "Holding/pending" state without a start date - which can now be done in 4.3 but that it can auto-activate the billing, either recurring or prepaid once the customer first logs into that user account.

This way a router can be pre-configured and shipped to the user, awaiting installation, having the service already setup without any billing being triggered until they plug the device in - but no requirement for the customer to call in to "activate" the service.