CustomShaderGUI.cs

using UnityEngine;
using UnityEditor;
using System;

public class CustomShaderGUI : ShaderGUI
{
	public override void OnGUI(MaterialEditor materialEditor, MaterialProperty[] properties)
	{
		// render the default gui
		base.OnGUI(materialEditor, properties);
		
		Material targetMat = materialEditor.target as Material;
		
		// see if redify is set, and show a checkbox
		bool redify = Array.IndexOf(targetMat.shaderKeywords, "REDIFY_ON") != -1;
		EditorGUI.BeginChangeCheck();
		redify = EditorGUILayout.Toggle("Redify material", redify);
		if (EditorGUI.EndChangeCheck())
		{
			// enable or disable the keyword based on checkbox
			if (redify)
				targetMat.EnableKeyword("REDIFY_ON");
			else
				targetMat.DisableKeyword("REDIFY_ON");
		}
	}
}