This is fixed in the latest Unity plug-in download with several caveats:
1. You have to use the WP8 DLL even for Windows Store builds: https://github.com/shephertz/App42_Unity3D_SDK/tree/master/2.4.1/App42-Unity3D-SDK-WP8.
2. You must use the "PlayerSettings.MetroCompilationOverrides.UseNetCore" option. Using "PlayerSettings.MetroCompilationOverrides.UseNetCorePartially" will build but crash when initializing.
3. Use the C#/XAML/8.1 version from Unity 4.3.4f1. Note: Unity will be dropping the D3D11 option in the future.
Thanks to the App42 team for getting this fixed quickly!
Rob Targosz, Bent Vector Studios