Unity 4.6 GUI - Dynamically change the functions called by GUI elements

There's a super simple way to change events:

Button button = buttonObject.GetComponent<Button>();
//Remove the existing events
button.onClick.RemoveAllListeners();
//Add your new event
button.onClick.AddListener(delegate {
            Debug.Log("Button " + button.name + " has been clicked!");
    });

Additionally, if you need more than just the events provided by default, I'd suggest instead attaching a EventTrigger to your game object. This gives us access to the BaseEventData object returned from the event, telling us stuff like the object that created the event. Then you can do something like:

//Create an event delegate that will be used for creating methods that respond to events
public delegate void EventDelegate(UnityEngine.EventSystems.BaseEventData baseEvent);

Then we can create a method for handling events, the signature must match that of our delegate. So, it needs to return void and accept BaseEventData as its first and only parameter:

public void DropEventMethod(UnityEngine.EventSystems.BaseEventData baseEvent) {
    Debug.Log(baseEvent.selectedObject.name + " triggered an event!");
    //baseEvent.selectedObject is the GameObject that triggered the event,
    // so we can access its components, destroy it, or do whatever.
}

Finally, to dynamically add the event:

//Get the event trigger attached to the UI object
EventTrigger eventTrigger = buttonObject.GetComponent<EventTrigger>();

//Create a new entry. This entry will describe the kind of event we're looking for
// and how to respond to it
EventTrigger.Entry entry = new EventTrigger.Entry();

//This event will respond to a drop event
entry.eventID = EventTriggerType.Drop;

//Create a new trigger to hold our callback methods
entry.callback = new EventTrigger.TriggerEvent();

//Create a new UnityAction, it contains our DropEventMethod delegate to respond to events
UnityEngine.Events.UnityAction<BaseEventData> callback =
    new UnityEngine.Events.UnityAction<BaseEventData>(DropEventMethod);

//Add our callback to the listeners
entry.callback.AddListener(callback);

//Add the EventTrigger entry to the event trigger component
    eventTrigger.delegates.Add(entry);