Nope, that makes sense to me too, I had originally misinterpreted your post as saying the graphics would need to be re-targeted and you were talking about everything else. :)
OpenGL ES + C won't fly on WP7. There's no OpenGL ES support (XNA is the exposed 3d API) and you can't write C as you're limited to the managed-only subset of .NET.
You could use LLVM to compile to .Net. But not sure how easy it is to wrap the OpenGL calls.
Not even worth thinking about... DX and OGL are mutually unintelligible :(
Actually, that's not true. WebGL on Windows is implemented using D3D (ANGLE converts OGL ES calls to D3D9) in most cases, and WINE has done D3D on top of OGL for years. They're very different, but translating between the two really isn't as big a deal as you'd think. (Just keep in mind that XNA isn't D3D -- it may backend to it, but it's a very different API.)