@FliegendeWurst would you be able to take a peek? Would be greatly appreciated, since you seem to have familiarity with cross-compilation issues.

Found a few things, but they won't fix this issue yet.

https://github.com/micropython/micropython/blob/master/ports/unix/Makefile#L34

uname -s needs to be replaced, see pkgs/by-name/ke/kew/package.nix for an example.

https://github.com/micropython/micropython/blob/master/ports/unix/Makefile#L154

pkg-config needs to be prefixed as ${stdenv.cc.targetPrefix}pkg-config (multiple instances in that file)

The rather more annoying issue is that this project vendors dozens of third-party libraries. All of them at an outdated version, of course.

In fact you got "lucky" by cross-compiling for riscv, when I try to build for aarch64 I get a false-positive notification about a logic bug in mbedtls...

Anyway, let's get to the real issue. The project contains "mpy-cross", used to turn scripts into precompiled bytecode. This compiler runs as part of the build process. I imagine package users also use it. So, to perform a cross build, we must get mpy-cross from the native build.
To do so, it would be easiest to patch https://github.com/micropython/micropython/blob/master/mpy-cross/mpy_cross/__init__.py#L64 to return a reference to ${lib.getExe' buildPackages.micropython "mpy-cross"}

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 407238 --package pkgsCross.aarch64-multiplatform.micropython --package pkgsCross.riscv64.micropython
Commit: e0008848a138e7ed784052591c945568317ac816

x86_64-linux

I will merge if @FliegendeWurst gives an ACK

@FliegendeWurst ping