Example of how to make editor classes in assemblies accessible in Unity (untested pseudo code).

MyProxy.cs

/////////////////////////////////////////////////////////////////
//	MyProxy.cs - the C# source file in your assets
/////////////////////////////////////////////////////////////////


using UnityEngine;
using UnityEditor;
using MyAssemblyNamespace;


[CustomEditor (typeof (MyBehaviour))]
public MyProxy : Editor
{
	MyInspector owner;
	
	
	public MyProxy ()
	{
		owner = new MyInspector (this);
	}
	
	
	public override void OnInspectorGUI ()
	{
		owner.OnInspectorGUI ();
	}
	
	/* ... */
}


/////////////////////////////////////////////////////////////////
//	MyInspector.cs - the actual inspector code built to assembly
/////////////////////////////////////////////////////////////////


using UnityEngine;
using UnityEditor;


public class MyInspector
{
	Editor proxy;
	
	
	public MyInspector (Editor proxy)
	{
		this.proxy = proxy;
	}
	
	
	public void OnInspectorGUI ()
	{
		GUILayout.Label ("KAN HAZ INSPECTAR?!");
		GUILayout.Label (string.Format ("Target: {0}", proxy.target == null ? "none" : proxy.target.ToString ()));
	}
	
	
	/* ... */
}