I think it's not as simple as just setting the attribute.

Here is a link
http://haacked.com/archive/2006/11/26/Adding_ClientSide_Custom_Properties_To_Controls.aspx

>Hi everybody,
>
>Do you know how can I add a custom property to a control?
>
>Here is the code I'm trying
>

>protected void Page_Load(object sender, EventArgs e)
>    {
>        string sConnString = ConfigurationManager.ConnectionStrings["FCCMSConnectionString"].ConnectionString;
>        SqlConnection oCn = new SqlConnection(sConnString);
>
>        SqlCommand Command = new SqlCommand("GetSchoolsBySiteID", oCn);
>        Command.CommandType = CommandType.StoredProcedure;
>
>        SqlParameter ParSiteID = new SqlParameter("@SiteID", this.GetParameter());
>
>        Command.Parameters.Add(ParSiteID);
>
>        Command.Connection.Open();
>
>        SqlDataReader oReader = Command.ExecuteReader();
>
>        while (oReader.Read())
>        {
>         ListItem li = new ListItem();
>           li.Attributes.Add("Phone", oReader["Phone"].ToString()) ;
>           li.Text = oReader["School"].ToString() ;
>           li.Value = li.Text;
>           this.ddlSchool.Items.Add(li);
>        }
>        oReader.Close();
>        oCn.Close();
>        this.ddlSchool.Attributes.Add("onchange","alert( this.options[this.selectedIndex].Phone);");
>    }
>

>
>However, JavaScript shows undefined all the time. Do you see what's wrong here or how can I create a new custom property and access it?
>
>Thanks in advance.