Short answer: You can do it today. The associated docs are on our priority backlog.
Long answer: Today, out of the box, the Breeze product ships with adapters for the ASP.NET Web API and for OData. It also ships with .NET components that interface with the Entity Framework and that generate Breeze metadata from an Entity Framework model; that EF model could be developed code first or database first.
The Breeze client is in no way limited to these technologies; they are merely the first backend components available; we’d be thrilled to help you adapt Breeze to your preferred server stack.