I have native Nix fork at https://github.com/volth/nixpkgs-windows https://github.com/volth/nix-windows (with MSVC and Perl as stdenv.shell because there is no bash working properly with Windows path names, all bashes require some sort of FHS emulation (moreover there no perl nor coreutils working properly with Windows long paths, build process of some native program use long paths which stdenv has to me able to cp -r and rm -rf at least). Perl + some 3rd party modules Win32::LongPath + patches not accepted by upstream was the closest shell to support all kinds of paths. Perhaps I had to use Powershell.
It builds Perl, Python, LLVM, Chromium, Win64Dbg, …
Mingw binary packages can be unpacked to c:\nix\store as-is, without recompilation or patchelf’ing, there is an updater script for them.
My goal was mainly to build Chromium with native Windows font rendering (free of freetype ugliness), so the development is stuck after the goal was achieved.