Can I get a prefab to render in front of GUI buttons created in script?

You could hide the buttons.

Put your GUI objects inside an empty Game Object, then, attach following C# script to it:

 public class TurnButtonsOnOFF : $$anonymous$$onoBehaviour {
 
     void Start () {
         turnChildrenOn();        
     }
     
     void turnChildrenOn () {
         foreach (Transform child in transform) {
             child.gameObject.SetActiveRecursively(true);
         }
     }
     
     void turnChildrenOff () {
         foreach (Transform child in transform) {
             child.gameObject.SetActiveRecursively(false);
         }    
     }    
 }

Now, manually disable the buttons object in the inspector.

That way, whenever you want to hide the buttons, all you have to do is call the script function turnChildrenOff, and vice versa.

The question is about OnGUI code, not game objects. It also wouldn't look good if you just turned the buttons off before fading to white.

Any OnGui code has to go in a GameObject... If you don’t want to turn them off, you can modify their alpha. Perhaps adding a Guistyle and changing the alpha value on normal/hover... :)

Or just use DrawTexture, as I mentioned in my answer. Also you can just use GUI.color to change alpha, you don't need to add styles or anything.