TextMarker.cs

using System.ComponentModel;
using UnityEngine;
using UnityEngine.Playables;
using UnityEngine.Timeline;

// テキスト情報を送信するマーカー
[System.Serializable, DisplayName("テキストのマーカー")]
public class TextMarker : Marker, INotification
{
    public string message;

    public PropertyName id
    {
        get
        {
            return new PropertyName("method");
        }
    }
}

TextMerkerReciever.cs

using UnityEngine;
using UnityEngine.Playables;
using UnityEngine.UI;

// テキスト情報を受信するマーカー
public class TextMerkerReceiver : MonoBehaviour, INotificationReceiver
{
    Text label;

    private void Awake()
    {
        label = GetComponent<Text>();
    }

    public void OnNotify(Playable origin, INotification notification, object context)
    {
        var element = notification as TextMarker;
        if (element == null)
            return;

        label.text = element.message;
    } 
}