I'm sorry I couldn't contribute to the discussion in time. The fix of the fstat() Win32 port looks good to me. I agree that there's a need for multiple fseek() ports to address the shortcomings of the MSVC functionality.
The documentation event states that "on devices incapable of seeking, the return value is undefined". A simple wrapper using GetFileType() or the new fstat(), to filter non-seekable devices before delegation, will probably suffice.
