configuration.nix

    packageOverrides = super: {
      xorg = super.xorg // rec {
        xkeyboard_config_dvp = super.pkgs.lib.overrideDerivation super.xorg.xkeyboardconfig (old: {
          patches = [
            (builtins.toFile "ru-dvp.patch" ''
                --- xkeyboard-config-2.16-orig/symbols/ru       2014-12-11 01:56:38.000000000 +0300
                +++ xkeyboard-config-2.16/symbols/ru    2016-04-15 14:27:25.075214654 +0300
                @@ -719,3 +719,24 @@
                     key <TLDE> { [  bracketright,  bracketleft  ] };
                     key <BKSL> { [   Cyrillic_io,  Cyrillic_IO  ] };
                 };
                +partial default alphanumeric_keys
                +xkb_symbols "dvp" {
                +    include "ru(common)"
                +
                +    name[Group1]= "Russia";
                +
                +    key <AE01> {        [       numerosign, percent   ]       };
                +    key <AE02> {        [       quotedbl,   7         ]       };
                +    key <AE03> {        [       question,   5         ]       };
                +    key <AE04> {        [       slash,      3         ]       };
                +    key <AE05> {        [       parenleft,  1         ]       };
                +    key <AE06> {        [       equal,      9         ]       };
                +    key <AE07> {        [       asterisk,   0         ]       };
                +    key <AE08> {        [       parenright, 2         ]       };
                +    key <AE09> {        [       plus,       4         ]       };
                +    key <AE10> {        [       minus,      6         ]       };
                +    key <AE11> {        [       exclam,     8         ]       };
                +    key <AE12> {        [       semicolon,  colon     ]       };
                +    key <AB10> {        [       period,     comma     ]       };
                +    key <BKSL> {        [       backslash,  bar       ]       };
                +};
              '')
          ];
        });
        xorgserver = super.pkgs.lib.overrideDerivation super.xorg.xorgserver (old: {
          postInstall = ''
            rm -fr $out/share/X11/xkb/compiled
            ln -s /var/tmp $out/share/X11/xkb/compiled
            wrapProgram $out/bin/Xephyr \
              --set XKB_BINDIR "${xkbcomp}/bin" \
              --add-flags "-xkbdir ${xkeyboard_config_dvp}/share/X11/xkb"
            wrapProgram $out/bin/Xvfb \
              --set XKB_BINDIR "${xkbcomp}/bin" \
              --set XORG_DRI_DRIVER_PATH ${super.mesa}/lib/dri \
              --add-flags "-xkbdir ${xkeyboard_config_dvp}/share/X11/xkb"
            ( # assert() keeps runtime reference xorgserver-dev in xf86-video-intel and others
              cd "$dev"
              for f in include/xorg/*.h; do # */
                sed "1i#line 1 \"${old.name}/$f\"" -i "$f"
              done
            )
          '';
        }); 
        setxkbmap = super.pkgs.lib.overrideDerivation super.xorg.setxkbmap (old: {
          postInstall =
            ''
              mkdir -p $out/share
              ln -sfn ${xkeyboard_config_dvp}/etc/X11 $out/share/X11
            '';
        });
        xkbcomp = super.pkgs.lib.overrideDerivation super.xorg.xkbcomp (old: {
          configureFlags = "--with-xkb-config-root=${xkeyboard_config_dvp}/share/X11/xkb";
        });
      };
    };
  };

Is this still up to date? Building xorgserver fails on ln -s /var/tmp $out/share/X11/xkb/compiled (the xkb folder doesn't exists, a mkdir fixes the xorgserver build). Once I could get it building, when trying to activate the added custom keymap setxkbmap fails.

No, you need to override xorg.xorgserver differently now, ... and there's also xkbvalidate now, that has to be overridden.

        xorgserver = super.xorg.xorgserver.overrideAttrs (old: {
          configureFlags = old.configureFlags ++ [
            "--with-xkb-bin-directory=${xkbcomp}/bin"
            "--with-xkb-path=${xkeyboardconfig_rolf}/share/X11/xkb"
          ];
        }); 

      xkbvalidate = super.xkbvalidate.override {
        libxkbcommon = super.libxkbcommon.override {
          xkeyboard_config = xorg.xkeyboardconfig_rolf;
        };
      };

... see http://stesie.github.io/2018/09/nixos-custom-keyboard-layout-revisited for a full example

Where should I get the original rules/ru file from?

Edit: Found it in /nix/store/xifpfl33xirk2ysc740lckj7lf0z6k42-xkeyboard-config-2.33.tar.bz2, but doing this kind of a modification seems to trigger re-compilation of Xorg and GNOME packages :(

Yeah, I didn't care about recompilation that much, as I was usually doing this on a dedicated beefy machine. But nowadays I've gave up on idea of software-based custom layouts - too many programs that I need are handling it very badly. And there is hardly anything that you can do about it.

I actually found out that the XKB version in NixOS 21.11 supports loading keyboard layouts from users' home directories! I just had to add this to my configuration.nix:

  environment.sessionVariables = rec {
    XDG_CONFIG_HOME = "\${HOME}/.config";
  };

and create correct ~/.config/xkb/rules/evdev, ~/.config/xkb/rules/evdev.xml and ~/.config/kxb/symbols/* files.

I have my customized Dvorak layout and two shortcuts with which I can activate either a standard layout or my custom one with one keypress.