That's likely the model I'd use. Charge customers a subscription fee for access to back end services, give them a free Win8 front end from the Store. That way, even if someone downloads the free app, they don't have access to the back end services until they pay the subscription fee... and when they stop paying the subscription fee/maintenance fee, they lose access to the service.

Businesses can bitch all they want, but they do it every day... with SalesForce.com; Google Apps; Office365