When running multiple warehouses, each warehouse is tied to a single variant. The typical setup is to enable/disable a variant  using the Availability rule based on the inventory available, geolocation, etc. In this case, currently the only way would be to run a scheduled task to automatically enable the fallback variant when the satellite variants are out of stock. This could be achieved using a script and the Revindex Task Scheduler  http://www.revindex.com/Product-Detail/revindex-task-scheduler