Attempting to override the default UnityEvent drawer.

Drawer.cs

using UnityEngine;
using UnityEditor;
using UnityEditorInternal;
using UnityEngine.Events;
using System.Reflection;


namespace Test
{
	[CustomPropertyDrawer (typeof (UnityEventBase), true)]
	public class Drawer : UnityEventDrawer
	{
		[InitializeOnLoadMethod]
		static void OnLoad ()
		{
			Debug.LogFormat ("Attempting to disable built-in property drawer");

			/*object drawer = typeof (UnityEventDrawer).GetCustomAttributes (true).FirstOrDefault (attribute => attribute is CustomPropertyDrawer);
			typeof (CustomPropertyDrawer).GetField ("m_Type", BindingFlags.Instance | BindingFlags.NonPublic).SetValue (drawer, null);*/

			ReplaceDrawer<UnityEventBase, Drawer> ();
		}


		static void ReplaceDrawer<TType, TDrawer> ()
			where TType : class
			where TDrawer : PropertyDrawer
		{
			MethodInfo dictionarySetter = ScriptAttributeUtility.DrawerTypeForTypeField.FieldType.GetMethod ("set_Item");
			dictionarySetter.Invoke (
				ScriptAttributeUtility.DrawerTypeForType,
				new []
				{
					typeof (TType),
					ScriptAttributeUtility.DrawerKeySet.Create<TType, TDrawer>()
				}
			);
		}


		public override void OnGUI (Rect position, SerializedProperty property, GUIContent label)
		{
			base.OnGUI (position, property, label);

			GUI.Label (position, "OVERRIDDEN");

			Debug.Log ("PING");
		}
	}
}

Reflected-ScriptAttributeUtility.cs

using System;
using System.Reflection;
using UnityEditor;


namespace Test
{
	public static class ScriptAttributeUtility
	{
		public static class DrawerKeySet
		{
			static Type s_Type;


			public static Type Type
			{
				get
				{
					return s_Type ?? (s_Type = ScriptAttributeUtility.Type.GetNestedType ("DrawerKeySet", BindingFlags.NonPublic));
				}
			}


			public static object Create<TType, TDrawer> ()
				where TType : class
				where TDrawer : PropertyDrawer
			{
				object instance = Activator.CreateInstance (Type);

				Type.GetField ("drawer").SetValue (instance, typeof (TDrawer));
				Type.GetField ("type").SetValue (instance, typeof (TType));

				return instance;
			}
		}


		static Type s_Type;
		static FieldInfo s_DrawerTypeForTypeField;
		static object s_DrawerTypeForType;


		public static Type Type
		{
			get
			{
				return s_Type ?? (s_Type = typeof (EditorWindow).Assembly.
					GetType ("UnityEditor.ScriptAttributeUtility", throwOnError: true, ignoreCase: false));
			}
		}


		public static FieldInfo DrawerTypeForTypeField
		{
			get
			{
				return s_DrawerTypeForTypeField ?? (s_DrawerTypeForTypeField = Type.
					GetField ("s_DrawerTypeForType", BindingFlags.Static | BindingFlags.NonPublic));
			}
		}


		public static object DrawerTypeForType
		{
			get
			{
				if ((s_DrawerTypeForType ?? (s_DrawerTypeForType = DrawerTypeForTypeField.GetValue (null))) != null)
				{
					return s_DrawerTypeForType;
				}

				Type.GetMethod ("BuildDrawerTypeForTypeDictionary", BindingFlags.NonPublic | BindingFlags.Static).Invoke (null, null);

				return s_DrawerTypeForType = DrawerTypeForTypeField.GetValue (null);
			}
		}
	}
}