While I’m confused as to why the boot would need to decide on a MAC address (VM or cheap SBC without a burned-in address? the first case might be easier to correct from the outside), the general state of NixOS is that some things are very flexible while others only cover some common cases (ones that the original author needed to solve). Unlike a traditional distro where the package manager will complain if you replace distro-provided stuff, in NixOS it’s entirely possible to override parts that don’t work for you rather than paper over them with programmatic overrides like these. It’s not even hard to upstream your changes if you make them backwards-compatible, although the benefit can be limited because the testing is not particularly thorough so other changes may still inadvertently break them.