nixpkgs already has infrastructure to compile to wasi the same way you compile to other platforms like arm, darwin (macOS), musl, etc.
nix-build -A pkgsCross.wasi32.$pkg
i haven't found any $pkg there that actually builds though. coreutils depends on posix stuff, busybox tries to include a non-existent netdb.h file. even hello barfs inside i think some autotools-generated wrapper around fcntl.
i don't understand enough about wasm to know if it really is reasonable to think of it as a "system" the same was x86_64-linux or aarch64-multiplatform is a "system", but if so i'd love the equivalent of this blog post showing how to use (or fix) the wasi32 system!