It doesn't sound like you'll have access to the Win32 API, so you'd have to translate all the Win32 API calls made by the x86 application into WinRT calls. And

It doesn't sound like you'll have access to the Win32 API, so you'd have to translate all the Win32 API calls made by the x86 application into WinRT calls. And will they actually let you sell an application through the Windows Store whose purpose is to run other non-Window-Store-acquired code? It seems likely that they'll have to institute a policy against such applications, just like the Apple App Store, to prevent end-runs around the Windows Store.