Ways to break up a text label into separate game objects?

I have a couple of ideas. Since I've never done this for "real," I cannot make a recommendation.

1) You could make invisible GUI buttons. Place normal buttons over all the text you want to be hot. Then you can define your own GUIStyle leaving out a texture for the 'Normal' state of the buttons. This solution has the added benefit that you can specify a texture 'Active' texture...quarter opacity perhaps...so the user gets a flash of a texture when the button is pressed as feedback.

2) Your mask idea could be implement as a GUI.DrawTexture and you could sample the color of the texture at the mouse cursor as feedback about the button. I played a bit with this idea:

 public class DetectColor : MonoBehaviour {

     public Texture2D tex;
     private Rect rect; 
     
     void Start() {
         rect = new Rect(100f, 100f, tex.width, tex.height);
     }
     
     void OnGUI () {
         Event e = Event.current;
         
         if (e.type == EventType.mouseDown) {
             if (rect.Contains (e.mousePosition)) {
                 int x = (int)(e.mousePosition.x - rect.x);
                 int y = tex.height - (int)(e.mousePosition.y - rect.y);
                 Color color = tex.GetPixel(x, y);
                 Debug.Log ("x-y ("+x+"-"+y+" color="+color);
             }
         }
     GUI.DrawTexture (rect, tex);
     }
 }

Note with this solution the opacity had to be non-zero for GetPixel to return correct values. Even the lightest value in the alpha channel and it worked.