I'll look at that. As for the prototype, I've tried a bunch of things, but here's where I left off yesterday.

{ sourceRoot, lib, pkgsCross, makeRustPlatform }:

pkgsCross.mingw32.callPackage
({ rustPlatform, buildPackages, pkg-config, cmake, wine }:
  buildPackages.rustPlatform.buildRustPackage (finalAttrs:
    let
      inherit (finalAttrs) src pname version meta buildInputs;
      manifest = lib.importTOML "${src}/ewext/Cargo.toml";
    in {
      pname = "ewext";
      inherit (manifest.package) version;

      src = sourceRoot;
      sourceRoot = "source/ewext";

      cargoLock.lockFile = "${src}/ewext/Cargo.lock";

      # cargoBuildTarget = "i686-pc-windows-gnu";

      strictDeps = true;
      nativeBuildInputs = [ pkg-config cmake ];

      nativeCheckInputs = [ wine ];
      doCheck = false;
      # buildInputs = with pkgsCross.mingw32; [ openssl ];

      env = { CARGO_TARGET_I686_PC_WINDOWS_GNU_RUNNER = "wine"; };

      # env = {
      #   # CARGO_TARGET_I686_PC_WINDOWS_GNU_LINKER = "${mingw32.stdenv.cc.targetPrefix}gcc";
      #   OPENSSL_DIR = "${lib.getDev pkgsCross.mingw32.openssl}";
      #   OPENSSL_LIB_DIR = "${lib.getLib pkgsCross.mingw32.openssl}/lib";
      #   OPENSSL_NO_VENDOR = 1;
      # };

      meta = { };
    })) { }

       >           /nix/store/asqlz348q8ln81iza5wv79wvrrqhfvxh-i686-w64-mingw32-binutils-2.44/bin/i686-w64-mingw32-ld: /build/rustc-1.89.0-src/build/x86_64-unknown-linux-gnu/stage1-std/i686-pc-windows-gnu/release/deps/libgimli-10feed5729a3b158.rlib(gimli-10feed5729a3b158.gimli.3fc3ea03a1049962-cgu.01.rcgu.o):gimli.3fc3ea03a1049962-cgu.01:(.text+0xa6): more undefined references to `_Unwind_Resume' follow
       >           collect2: error: ld returned 1 exit status
       >
       >   = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
       >   = note: use the `-l` flag to specify native libraries to link
       >   = note: use the `cargo:rustc-link-lib` directive to specify the native libraries to link with Cargo (see https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-link-lib)
       >
       > error: could not compile `std` (lib) due to 1 previous error