They have just announced the availability of this feature for the .NET backend.
This is the announcement - http://azure.microsoft.com/blog/2014/10/02/custom-login-scopes-single-sign-on-new-asp-net-web-api-updates-to-the-azure-mobile-services-net-backend/
Take a look at Custom Scopes for Social Authentication Providers - I believe that this is the information that you are looking for.