Custom PowerUI Live HTML with a small mod/edit to add support for NodeCanvas's Dialogue Trees.

DialoguePowerUI.cs

using UnityEngine;
using System.Collections;
using PowerUI;
using NodeCanvas.DialogueTrees;

public class DialoguePowerUI : UnityEngine.MonoBehaviour {

	public TextAsset HtmlFile;
	#if UNITY_EDITOR
	private LiveHtml Reloader;
	#endif

	private IDialogueActor currentActor;
	private MultipleChoiceRequestInfo currentOptions;

	private Element Dialogue;
	private Element Choices;

	private static System.Action<int> Choice;
	
	// OnEnable is called when the game starts, or when the manager script component is enabled.
	void OnEnable () {
		print ("UI Enabled.");
		// Optional. This allows PowerUI to easily work with screens of varying resolution.
		UI.Resolution=new ResolutionInfo(1f);

		if(HtmlFile==null){
			Debug.Log("Please provide a HTML file for your UI.");
			#if UNITY_EDITOR
			Reloader.Stop();
			Reloader=null;
			#endif
		}else{
			// Write the html:
			UI.Html=HtmlFile.text;
			#if UNITY_EDITOR
			// Get the full asset path:
			string fullPath=UnityEditor.AssetDatabase.GetAssetPath(HtmlFile);
			// Create the loader:
			Reloader=new LiveHtml(fullPath,OnHtmlChanged);
			#endif
		}

		DialogueTree.OnDialogueStarted       += OnDialogueStarted;
		DialogueTree.OnDialoguePaused        += OnDialoguePaused;
		DialogueTree.OnDialogueFinished      += OnDialogueFinished;
		DialogueTree.OnSubtitlesRequest      += OnSubtitlesRequest;
		DialogueTree.OnMultipleChoiceRequest += OnMultipleChoiceRequest;

		Dialogue = UI.document.getElementByAttribute ("class", "Dialogue");
		Choices = UI.document.getElementByAttribute("class","Choices");
	}
	
	/// <summary>Called when the .html file is saved.</summary>
	void OnHtmlChanged(string path,string html){
		print ("UI Changed.");
		UI.Html=html;
	}
	
	// OnDisable is called when the manager script component is disabled. You don't need this.
	void OnDisable () {
		print ("UI Disabled.");
		UI.Destroy();
		#if UNITY_EDITOR
		Reloader.Stop();
		Reloader=null;
		#endif
	}

	void OnDialogueStarted(DialogueTree dialogue){
		print ("On Dialogue Started.");
		//We could do something here...
	}
	
	void OnDialoguePaused(DialogueTree dialogue){
		print ("On Dialogue Paused.");
		OnDialogueFinished(dialogue);
	}
	
	void OnDialogueFinished(DialogueTree dialogue){
		print ("On Dialogue Over.");
		if (currentActor != null)
			currentActor.speech = null;
		currentOptions = null;
		Dialogue.innerHTML = "";
	}
	
	
	//Function with same name as the event is called when the event is dispatched by the Dialogue Tree
	void OnSubtitlesRequest(SubtitlesRequestInfo info){
		print ("Subtitles Requested.");
		currentActor = info.actor;
		StatementProcessor.ProcessStatement(info.statement, info.actor, info.Continue);
		print ("Subtitles Requested.");
		Dialogue.innerHTML = info.actor.name + ":" + info.statement;
	}
	
	//A function with the same name as the subscribed Event is called when the event is dispatched
	void OnMultipleChoiceRequest(MultipleChoiceRequestInfo optionsInfo){
		print ("Multiple Choice Request.");
		currentOptions = optionsInfo;
		if (currentOptions.options.Count > 0) {
			Choices.innerHTML = "";
			foreach (var option in currentOptions.options) {
				Element ele=UI.document.createElement("<input type='button' onclick='DialoguePowerUI.ClickedOption' id='" + option.Value + "' value=''/>");
				ele["value"] = option.Key.ToString();
				Choices.AppendNewChild(ele);
			}
			Choice = currentOptions.SelectOption;
			currentOptions = null;
		}
	}

	public static void ClickedOption(UIEvent e){
		print ("Button Clicked.");
		int c = int.Parse (e.target ["id"]);
		Choice (c);
		UI.document.getElementByAttribute ("class", "Choices").innerHTML = "";
		return;
	}
}

GUI.html

<!-- Time for a little CSS! -->
<style type='text/css'>

.Dialogue {
	text-align: center;
	height: 100%;
	width: 100%;
	v-align: middle;
}

.Choices {
	position: absolute;
	bottom: 0px;
	left: 0px;
	width: 100%;
	height: 100px;
}

</style>

<div class='Dialogue'>
</div>

<div class='Choices'>
</div>