There are a number of physics/games engines being used within academia and with the release of Windows 8 the following video discusses how to consume a non-WinRT C++ Code set see: Building Windows Runtime Components with C++.
Box2D is open source physics engine, and it is used in number of solutions including Physics Helper, coco2dx and Construct2 by Scirra.com.
For the Unreal Engine article see: http://epicgames.com/news/unreal-engine-3-debuts-on-windows-8, available for Windows RT at http://www.unrealengine.com/en/platforms/
The Autodesk Scaleform 4.1 states that it does support Windows 8 Metro in the 4.1 version, http://gameware.autodesk.com/2013-features/, you can download the tool for non-commercial use.
Other really useful tools
UniSwf simply publish assets from Flash and they seamlessly appear in the Unity editor. see http://www.uniswf.com