This is an updated version of ProGM's script. Now it handles Vector2, arrays and nested fields as well. You can also drag points locally. See the original code here: https://gist.github.com/ProGM/226204b2a7f99998d84d755ffa1fb39a. See also helper methods here: http://answers.unity3d.com/questions/425012/get-the-instance-the-serializedproperty-bel…

DraggablePointAttribute.cs

using UnityEngine;

public class DraggablePointAttribute : PropertyAttribute {
    public bool local;

    public DraggablePointAttribute (bool local = false) {
        this.local = local;
    }
}

DraggablePointAttributeDrawer.cs

using UnityEngine;
using UnityEditor;
using System;
using System.Collections;
using System.Reflection;
using System.Linq;

[CustomEditor(typeof(MonoBehaviour), true)]
public class DraggablePointAttributeDrawer : Editor {
    private readonly GUIStyle style = new GUIStyle();

    private Transform transform;

    private void OnEnable () {
        style.fontStyle = FontStyle.Bold;
        style.normal.textColor = Color.white;

        transform = (target as MonoBehaviour).transform;
    }

    public void OnSceneGUI () {
        if (Application.isPlaying) return;
        SerializedProperty property = serializedObject.GetIterator();
        CheckProperty(property);
    }

    private void CheckProperty (SerializedProperty property) {
        while (property.Next(true)) {
            if (property.propertyType == SerializedPropertyType.Vector2 || property.propertyType == SerializedPropertyType.Vector3) {
                FieldInfo field = GetParent(property).GetType().GetField(property.name);
                if (field == null) continue;
                object[] draggablePoints = field.GetCustomAttributes(typeof(DraggablePointAttribute), false);
                if (draggablePoints.Length > 0) {
                    DraggablePointAttribute attribute = draggablePoints[0] as DraggablePointAttribute;
                    DrawPoint(property, attribute.local, property.displayName);
                }
            }
            else if (property.propertyType == SerializedPropertyType.Generic && property.isArray) {
                FieldInfo field = GetParent(property).GetType().GetField(property.name);
                if (field == null) continue;
                object[] draggablePoints = field.GetCustomAttributes(typeof(DraggablePointAttribute), false);
                if (draggablePoints.Length > 0) {
                    DraggablePointAttribute attribute = draggablePoints[0] as DraggablePointAttribute;
                    for (int i = 0; i < property.arraySize; i++) {
                        SerializedProperty element = property.GetArrayElementAtIndex(i);
                        if (element.propertyType == SerializedPropertyType.Vector2 || element.propertyType == SerializedPropertyType.Vector3) {
                            DrawPoint(element, attribute.local, property.displayName + " " + i);
                        }
                    }
                }
            }
        }
    }

    private void DrawPoint (SerializedProperty property, bool local, string name) {
        if (property.propertyType == SerializedPropertyType.Vector2) {
            Vector2 drawPos = local ? (Vector2)transform.TransformPoint(property.vector2Value) : property.vector2Value;
            Handles.Label(drawPos, name, style);
            EditorGUI.BeginChangeCheck();
            Vector2 position = Handles.PositionHandle(drawPos, Quaternion.identity);
            if (EditorGUI.EndChangeCheck()) {
                if (local) position = transform.InverseTransformPoint(position);
                property.vector2Value = position;
                serializedObject.ApplyModifiedProperties();
            }
        }
        else {
            Vector3 drawPos = local ? transform.TransformPoint(property.vector3Value) : property.vector3Value;
            Handles.Label(drawPos, name, style);
            EditorGUI.BeginChangeCheck();
            Vector3 position = Handles.PositionHandle(drawPos, Quaternion.identity);
            if (EditorGUI.EndChangeCheck()) {
                if (local) position = transform.InverseTransformPoint(position);
                property.vector3Value = position;
                serializedObject.ApplyModifiedProperties();
            }
        }
    }

    public object GetParent (SerializedProperty prop) {
        string path = prop.propertyPath.Replace(".Array.data[", "[");
        object obj = prop.serializedObject.targetObject;
        string[] elements = path.Split('.');
        foreach (string element in elements.Take(elements.Length - 1)) {
            if (element.Contains("[")) {
                string elementName = element.Substring(0, element.IndexOf("["));
                int index = Convert.ToInt32(element.Substring(element.IndexOf("[")).Replace("[", "").Replace("]", ""));
                obj = GetValue(obj, elementName, index);
            }
            else {
                obj = GetValue(obj, element);
            }
        }
        return obj ?? prop.serializedObject.targetObject;
    }

    public object GetValue (object source, string name) {
        if (source == null)
            return null;
        Type type = source.GetType();
        FieldInfo f = type.GetField(name, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance);
        if (f == null) {
            PropertyInfo p = type.GetProperty(name, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance | BindingFlags.IgnoreCase);
            if (p == null)
                return null;
            return p.GetValue(source, null);
        }
        return f.GetValue(source);
    }

    public object GetValue (object source, string name, int index) {
        IEnumerable enumerable = GetValue(source, name) as IEnumerable;
        if (enumerable == null) return null;
        IEnumerator enm = enumerable.GetEnumerator();
        while (index-- >= 0)
            enm.MoveNext();
        return enm.Current;
    }
}

Example.cs

using UnityEngine;

public class Platform : MonoBehaviour {

    [DraggablePoint(true)]
    public Vector3 startPoint;
    
    public PlatformMovement platformMovement;
    
    [System.Serializable]
    public class PlatformMovement {
        [DraggablePoint(true)]
        public Vector2[] localWaypoints;
    }
}