Unity : callbacks in component based design

I see two solutions to your questions.

1) Use the SendMessage(string methodName) function. It will call the given method on every components of the gameObject. (More info here: http://docs.unity3d.com/ScriptReference/Component.SendMessage.html).

Here is an example:

public class Script1 : MonoBehavior
{
    public void DestroyShip()
    {
        /* Destroy the ship here. */
        SendMessage("OnShipDestroyed");
    }
}

public class Script2 : MonoBehavior
{
    public void OnShipDestroyed()
    {
        this.ShowPoints();
    }

    public void ShowPoint()
    {
        /* Show points here. */
    }
}

2) Use .Net events. Receiver registers to an event triggered when the ship is destroyed. Here is an example as well:

public class Script1 : MonoBehavior
{
    public event System.EventHandler OnShipDestroyed;

    public void DestroyShip()
    {
        if (this.OnShipDestroyed != null)
        {
            this.OnShipDestroyed.Invoke(this, new System.EventArgs());
        }
    }
}

public class Script2 : MonoBehavior
{
    void Awake()
    {
        this.GetComponent<Script1>().OnShipDestroyed += OnShipDestroyed;
    }

    private void OnShipDestroyed
    {
        this.ShowPoint();
    }

    public void ShowPoint()
    {
        /* Show points here. */
    }
}

Note that this solution creates a dependency of Script2 to Script1.

Hope it helps.

I'm going to try and write some example code using delegates since I think they might do what you want. Code is mostly taken from the delegate section of MSDN

using UnityEngine;
using System.Collections;

namespace MyNamespace
{
    public class ShowPoints : MonoBehaviour
    {

        public void showPoitsn()
        {
            Debug.Log("ShowingPoints");
        }
    }
}

using UnityEngine;
using System.Collections;


namespace MyNamespace
{
    public class DestroyShip : MonoBehaviour
    {


        public void Destroy()
        {
            Debug.Log("ShipDestroyed");
        }
    }
}
using UnityEngine;
using System.Collections;

namespace MyNamespace
{
   public enum BehaviourType
    {
        JustDestroy,
        DestroyAndShowPoints,
        ShowPoints
    }
    public class DelegateHandler : MonoBehaviour
    {
        private DestroyShip dShip;
        private ShowPoints sPoints;
        public  BehaviourType type;
        delegate void MyDelegate();
        MyDelegate destroy, showPoints,c;
        void Start()
        {
            dShip = GetComponent<DestroyShip>();
            sPoints = GetComponent<ShowPoints>();

            destroy = new MyDelegate(dShip.Destroy);
            showPoints = new MyDelegate(sPoints.showPoitsn);

            ComposeDelegates();
        }
        //Now we need some logic to handle the composing of a new delegate that can invoke the functions depending on our cases

        void ComposeDelegates()
        {
           switch(type)
            {
                case BehaviourType.JustDestroy:
                    c = new MyDelegate(destroy);
                    break;
                case BehaviourType.DestroyAndShowPoints:
                    c = new MyDelegate(destroy + showPoints);
                    break;
                case BehaviourType.ShowPoints:
                    c = new MyDelegate(showPoints);
                    break;
            }

            c();

        }

    }
}

How this works is that you attach all the components to the gameObject and the select what type of behaviour you want from the DelegateHandler. In my opinion this covers your criteria and its easy to extend. Not sure about the dependancy criteria you have.