Neo2

The Neo2 keyboard layout can be set in the NixOS config via

{
 # ...
 services.xserver.xkb = {
    layout = "de";
    variant = "neo"; 
  };
  console.useXkbConfig = true;  # this enables Neo2 also in your TTYs.
 # ...
}

For peculiarities of Neo2 as a tty layout, see also the readme file /etc/kbd/keymaps/i386/neo/README.md. A one-off way to enable Neo2 in a running tty is

sudo loadkeys neo

With this, also the Mod4 layer should work immediately.