Hi Ruben,
This has been done using HTML5, OpenFL + HTML5 support has been improving, so although it is not as good as native, this is an existing option. Another development is that the ANGLE project (which implements OpenGL over DirectX, which is required for Metro apps) has been integrated into SDL, which we use, for desktop support, so that means that we may be able to "flip a switch" and have DirectX support in order to work as a Metro app.
Additional work involved would be to make sure that we can compile to WinRT, additional platform specifics (for accessing file paths, etc) as well as sorting out how we do the tool side of packaging, installing and running applications automatically. Unfortunately on this front, I've tried asking Microsoft evangelists many times about this, and never gotten a good answer.
So a good push could turn it from HTML5 to native