control where markup should appear on rendered page?

Question

I am building a Extender control. This control derives from the normal Control. Its purpose is to inject some text before/after the specified target control. I imagine this to be similar to how the AJAX Calendar Extender works. So it has the property like TargetControlID, etc etc.

I am not sure which event of the target control to hookup so that I can insert my comment before/after the control renders.

To pictorially represent what I am trying to accomplish:

I just want to put comments where the green dashes appear. Anyway I can control that...?

The source code I have tried working with so far...

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Text;
using System.IO;

namespace DropDownSubstitute
{
    /// <summary>
    /// 
    /// </summary>
    public class DropDownListExtender : Control
    {
        public string TargetControlID { get; set; }
        System.Web.UI.WebControls.DropDownList _ddl;
        protected override void OnInit(EventArgs e)
        {
            _ddl = this.Page.FindControl(this.TargetControlID) as DropDownList;
            if(_ddl == null) 
                throw new InvalidOperationException("TargetControlID for DropDownListExtender must be a valid ASP.NET DropDownList control");
            _ddl.PreRender += new EventHandler(RenderDropDown);

        }

        void RenderDropDown(object sender, EventArgs e)
        {
                //Desperately wanting to put a comment before the DropDownList renders
                // to the response 
        }

    }

}