Filling image without compromises

I'm building a progressive image that fills himself with time. Basically the image is, very simply, 2 half images, one above the other. It would be very simple to do that with a GUITexture on bottom, and a clipped texture drawn with GUI.DrawTextureWithTexCoords, but I need to draw things above it.

I know that I can invoke directly Camera.Render from OnGUI, but I have a very complex hierarchy of cameras, and they aren't always active, sometimes they are even instantiated, so it is a very complex thing to do.

There are three solutions, as far as I know, to achieve the goal and don't twist my existing code and scene completely.

1. Build a RenderTexture

2. Convert all the GUI layer I have in a polygonal gui with a dedicated camera

3. Alter the texture with Texture2D.SetPixels

All the solutions seems to be too slow or complex to achieve for this simple clipping operation.

Is there a way to solve the problem simply and fast?

EDIT: That's a simple script that works (except for the texture that is drawn in front of all the other things). I hope it helps to understand the goal I'm trying to achieve.

 using UnityEngine;
 
 public class ButtonOverlay: MonoBehaviour
 {
     public Texture2D texture;
 
     public float fill = 0.5f;
 
     protected GUITexture parentGUI;
 
     protected void Awake()
     {
         parentGUI = GetComponentInParent<GUITexture>();
     }
 
     protected void OnGUI()
     {
         var rect = parentGUI.GetScreenRect();
         rect.y   = Screen.height - rect.y - rect.height;
 
         var f = Mathf.Clamp(fill, 0, 1);
 
         var delta = rect.height * (1 - f);
 
         var clippedRect     = rect;
         clippedRect.y      += delta;
         clippedRect.height -= delta;
 
         var textureRect = new Rect(0, 0, 1, f);
 
         GUI.DrawTextureWithTexCoords(clippedRect, texture, textureRect);
     }
 }