GameObjectのヘッダーに情報を追加できるようにするエディタ拡張。

EditorHeaderHideFlags.cs

using System;
using System.Reflection;
using UnityEditor;
using UnityEngine;

[CustomEditor(typeof(GameObject))]
public class EditorHeaderHideFlags : Editor
{
    Editor defaultCustomEditor;

    Type defaultInspectorType;
    MethodInfo defaultHeaderGUI;

    GUIStyle inspectorBig;
    GUIStyle postLargeHeaderBackground;

    public EditorHeaderHideFlags()
    {
        // GameObjectのヘッダー描写はここが行っている
        defaultInspectorType = Assembly.GetAssembly(typeof(Editor)).GetType("UnityEditor.GameObjectInspector");
        // ヘッダー部分の呼び出しを取得
        defaultHeaderGUI = defaultInspectorType.GetMethod("OnHeaderGUI", BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
    }

    protected override void OnHeaderGUI()
    {
        inspectorBig = inspectorBig ?? new GUIStyle("In BigTitle");
        postLargeHeaderBackground = postLargeHeaderBackground ?? new GUIStyle("IN BigTitle Post");

        // カスタムエディター情報を取得
        defaultCustomEditor = CreateEditor(target, defaultInspectorType);
        // デフォルトの表示を描画
        defaultHeaderGUI.Invoke(defaultCustomEditor, null);

        // 空いているスペース分を戻す
        GUILayout.Space(
            -1f
            - inspectorBig.margin.bottom
            - inspectorBig.padding.bottom
            - inspectorBig.overflow.bottom
        );

        // ヘッダー追加項目を以下に記載
        using (new EditorGUILayout.VerticalScope(postLargeHeaderBackground, GUILayout.ExpandWidth(true)))
        {
            var obj = target as GameObject;

            bool hierarchyMode = (obj.scene.name == UnityEditor.SceneManagement.EditorSceneManager.GetActiveScene().name);
            // PrefabではHideFlagsを編集しても戻るため表示させない
            if (!PrefabUtility.GetCorrespondingObjectFromOriginalSource(target) && hierarchyMode)
            {
                using (var check = new EditorGUI.ChangeCheckScope())
                {
                    var hideFlags = obj.hideFlags;
                    using (new EditorGUILayout.HorizontalScope())
                    {
                        EditorGUIUtility.labelWidth = 60.0f;

                        GUILayout.Space(20);
                        hideFlags = (HideFlags)EditorGUILayout.EnumFlagsField("HideFlags", obj.hideFlags, GUILayout.Width(220.0f));

                        EditorGUIUtility.labelWidth = 0f;
                    }

                    if (check.changed)
                    {
                        Undo.RegisterCompleteObjectUndo(target, "Changed HideFlags");
                        obj.hideFlags = hideFlags;

                        var scene = UnityEditor.SceneManagement.EditorSceneManager.GetActiveScene();
                        UnityEditor.SceneManagement.EditorSceneManager.MarkSceneDirty(scene);
                    }
                }
            }
        }

    }

    public override void OnInspectorGUI() { }
}