Here's the diff I'm currently testing:

diff --git a/pkgs/os-specific/linux/plymouth/default.nix b/pkgs/os-specific/linux/plymouth/default.nix
index f2abfa68682..42ba23901e0 100644
--- a/pkgs/os-specific/linux/plymouth/default.nix
+++ b/pkgs/os-specific/linux/plymouth/default.nix
@@ -21,6 +21,8 @@ stdenv.mkDerivation rec {
   pname = "plymouth";
   version = "unstable-2023-02-22";
 
+  outputs = [ "out" "dev" ];
+
   src = fetchFromGitLab {
     domain = "gitlab.freedesktop.org";
     owner = pname;
@@ -30,7 +32,6 @@ stdenv.mkDerivation rec {
   };
 
   patches = [
-    ./add-option-for-installation-sysconfdir.patch
     ./dont-create-broken-symlink.patch
   ];
 
@@ -55,7 +56,6 @@ stdenv.mkDerivation rec {
   ];
 
   mesonFlags = [
-    "--sysconfdir=/etc"
     "-Dlogo=/etc/plymouth/logo.png"
     "-Dbackground-color=0x000000"
     "-Dbackground-start-color-stop=0x000000"
@@ -74,6 +74,10 @@ stdenv.mkDerivation rec {
 
   postPatch = ''
     substituteInPlace meson.build \
+      --replace "'PLYMOUTH_THEME_PATH', plymouth_theme_path" "'PLYMOUTH_THEME_PATH', '/etc/plymouth/themes'" \
+      --replace "'PLYMOUTH_PLUGIN_PATH', plymouth_plugin_path" "'PLYMOUTH_PLUGIN_PATH', '/etc/plymouth/plugins/'" \
+      --replace "'PLYMOUTH_POLICY_DIR', plymouth_policy_dir" "'PLYMOUTH_POLICY_DIR', '/etc/plymouth/'" \
+      --replace "'PLYMOUTH_CONF_DIR', plymouth_conf_dir" "'PLYMOUTH_CONF_DIR', '/etc/plymouth/'" \
       --replace "run_command(['scripts/generate-version.sh'], check: true).stdout().strip()" "'${version}'"
 
     sed -i '/^install_emptydir/d' src/meson.build

In reply to @kranzes:matrix.org
Are the unnecessary things other meson flags?

In reply to @elvishjerricco:matrix.org
That's interesting... I see output from the openvpn command before I see systemd[1]: Starting openvpn.service...

In reply to @elvishjerricco:matrix.org
But NixOS supports two different dbus implementations so that makes it trickier to implement that