概要

using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
public static class Example
{
    static Example()
    {
        SceneViewCameraWindow.additionalSettingsGui += OnGUI;
    }

    private static void OnGUI( SceneView sceneView )
    {
        GUILayout.Label( "ピカチュウ" );
    }
}

参考サイト様