A relative simple way to get the instance object from a custom property drawer that draws a serialized class inside a component

Example.cs

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour
{
	public ExampleClass example;

	void Start() {}
	void Update() {}
}

ExampleClass.cs

using UnityEngine;
using System;

[Serializable]
public class ExampleClass
{
	public string field = "example string";

	public void Print()
	{
		Debug.Log("ExampleClass.field: " + field);
	}
}

ExampleProperty.cs

using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer (typeof(ExampleClass))]
public class ExampleProperty : NestablePropertyDrawer
{
	protected new ExampleClass propertyObject { get { return (ExampleClass)base.propertyObject; } }
	private SerializedProperty stringField = null;

	protected override void Initialize(SerializedProperty prop)
	{
		base.Initialize(prop);

		if (stringField == null)
			stringField = prop.FindPropertyRelative("field");
	}

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

		EditorGUI.BeginProperty(position, label, prop);
		EditorGUI.BeginChangeCheck();
		EditorGUI.PropertyField(position, stringField);
		if (EditorGUI.EndChangeCheck())
		{
			stringField.serializedObject.ApplyModifiedProperties();
			propertyObject.Print();
		}
		EditorGUI.EndProperty();
	}
}

NestablePropertyDrawer.cs

using UnityEngine;
using UnityEditor;
using System;
using System.Collections;
using System.Reflection;
using System.Text.RegularExpressions;

public class NestablePropertyDrawer : PropertyDrawer
{
	private bool initialized = false;
	protected object propertyObject = null;
	protected Type objectType = null;
	
	private static readonly Regex matchArrayElement = new Regex(@"^data\[(\d+)\]$");
	protected virtual void Initialize(SerializedProperty prop)
	{
		if (initialized)
			return;

		SerializedObject serializedObject = prop.serializedObject;
		string path = prop.propertyPath;
	
		propertyObject = serializedObject == null || serializedObject.targetObject == null ? null : serializedObject.targetObject;
		objectType = propertyObject == null ? null : propertyObject.GetType();
		if (!string.IsNullOrEmpty(path) && propertyObject != null)
		{
			string[] splitPath = path.Split('.');
			Type fieldType = null;

			//work through the given property path, node by node
			for (int i = 0; i < splitPath.Length; i++)
			{
				string pathNode = splitPath[i];

				//both arrays and lists implement the IList interface
				if (fieldType != null && typeof(IList).IsAssignableFrom(fieldType))
				{
					//IList items are serialized like this: `Array.data[0]`
					Debug.AssertFormat(pathNode.Equals("Array", StringComparison.Ordinal), serializedObject.targetObject, "Expected path node 'Array', but found '{0}'", pathNode);

					//just skip the `Array` part of the path
					pathNode = splitPath[++i];

					//match the `data[0]` part of the path and extract the IList item index
					Match elementMatch = matchArrayElement.Match(pathNode);
					int index;
					if (elementMatch.Success && int.TryParse(elementMatch.Groups[1].Value, out index))
					{
						IList objectArray = (IList)propertyObject;
						bool validArrayEntry = objectArray != null && index < objectArray.Count;
						propertyObject = validArrayEntry ? objectArray[index] : null;
						objectType = fieldType.IsArray
							? fieldType.GetElementType()          //only set for arrays
							: fieldType.GenericTypeArguments[0];  //type of `T` in List<T>
					}
					else
					{
						Debug.LogErrorFormat(serializedObject.targetObject, "Unexpected path format for array item: '{0}'", pathNode);
					}
					//reset fieldType, so we don't end up in the IList branch again next iteration
					fieldType = null;
				}
				else
				{
					FieldInfo field;
					Type instanceType = objectType;
					BindingFlags fieldBindingFlags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.FlattenHierarchy;
					do
					{
						field = instanceType.GetField(pathNode, fieldBindingFlags);

						//b/c a private, serialized field of a subclass isn't directly retrievable,
						fieldBindingFlags = BindingFlags.Instance | BindingFlags.NonPublic;
						//if neccessary, work up the inheritance chain until we find it.
						instanceType = instanceType.BaseType;
					}
					while (field == null && instanceType != typeof(object));

					//store object info for next iteration or to return
					propertyObject = field == null || propertyObject == null ? null : field.GetValue(propertyObject);
					fieldType = field == null ? null : field.FieldType;
					objectType = fieldType;
				}
			}
		}
		initialized = true;
	}

	public override void OnGUI(Rect position, SerializedProperty prop, GUIContent label)
	{
		Initialize(prop);
	}
}

Yeah, I remember doing a similar thing for a project at some point. I think in my case it was about multi object editing, which also reuses the same drawer instance. But I guess I'll not update this gist to support that.