I've started a community project to implement the ASP.NET vNext authorization scheme in the current 4.6.1 release. The project is on GitHub: https://github.com/pschaeflein/mvc5-authorization-policy . An early version is in the NuGet gallery: https://www.nuget.org/packages/MVC5AuthZPolicy/
mvc5-authorization-policy
Authorization concepts from ASP.NET Core back-ported to ASP.NET 4.6 / MVC 5