I had to do some minor changes (update to the latest version), but it seems to have worked. For reference, here's what I ended up with:

{ stdenv, libuuid, libxcb, expat, gtk3, glib }:

stdenv.mkDerivation {
  pname = "Zebralette-mini-Zebra";
  version = "2.9.3";
  src = builtins.fetchurl {
    url = "https://dl.u-he.com/releases/Zebra2_293_12092_Linux.tar.xz";
    sha256 = "sha256:1imn0a1pjgqihcms86m7wblc9fifmc2l9xczykl8c0bkzw883xpx";
  };
  dontBuild = true;
  sourceRoot = "Zebra2-12092";

  installPhase = ''
    mkdir -p $out/lib/vst
    cp -r Zebra2/* $out/lib
    ln -f $out/lib/Zebra2.64.so $out/lib/vst/Zebra2.64.so
  '';

  postFixup = ''
    patchelf \
      --set-rpath ${expat}/lib:${libxcb}/lib:${libuuid.out}/lib \
      $out/lib/vst/Zebra2.64.so
    patchelf \
      --set-interpreter $(cat $NIX_CC/nix-support/dynamic-linker) \
      --set-rpath ${gtk3}/lib:${glib.out}/lib \
      $out/lib/dialog.64
  '';
}

However, when trying to run the plugins (in Bitwig), I'm getting these messages:

I had to do some minor changes (update to the latest version), but it seems to have worked. For reference, here's what I ended up with:

{ stdenv, libuuid, libxcb, expat, gtk3, glib }:

stdenv.mkDerivation {
  pname = "Zebralette-mini-Zebra";
  version = "2.9.3";
  src = builtins.fetchurl {
    url = "https://dl.u-he.com/releases/Zebra2_293_12092_Linux.tar.xz";
    sha256 = "sha256:1imn0a1pjgqihcms86m7wblc9fifmc2l9xczykl8c0bkzw883xpx";
  };
  dontBuild = true;
  sourceRoot = "Zebra2-12092";

  installPhase = ''
    mkdir -p $out/lib/vst
    cp -r Zebra2/* $out/lib
    ln -f $out/lib/Zebra2.64.so $out/lib/vst/Zebra2.64.so
  '';

  postFixup = ''
    patchelf \
      --set-rpath ${expat}/lib:${libxcb}/lib:${libuuid.out}/lib \
      $out/lib/vst/Zebra2.64.so
    patchelf \
      --set-interpreter $(cat $NIX_CC/nix-support/dynamic-linker) \
      --set-rpath ${gtk3}/lib:${glib.out}/lib \
      $out/lib/dialog.64
  '';
}

However, when trying to run the plugins (in Bitwig), I'm getting the above error (Installation issue: no GUI resources. Please run the installer again). Does that sound familiar?