This post by Fredrik Normén is a big help. I need to create a custom parameter that binds to Context. I was able to create in minutes following the example. The result is:

    public class ContextParameter : System.Web.UI.WebControls.Parameter
    {
        private string _contextField;

        public ContextParameter()
        {
        }

        protected ContextParameter(ContextParameter original)
            : base(original)
        {
            this.ContextField = original.ContextField;
        }

        protected override object Evaluate(HttpContext context, Control control)
        {
            if (context != null && _contextField != null)
            {
                return context.Items[_contextField];
            }
            else
            {
                return null;
            }
        }

        protected override Parameter Clone()
        {
            return new ContextParameter(this);
        }


        public string ContextField
        {
            get
            {
                return _contextField;
            }
            set
            {
                _contextField = value;
            }
        }
    }

I can the use the parameter like:

<dt:ContextParameter Name="PageID" ContextField="PageID" Type="String" DefaultValue="136b08ff-3752-4704-8db8-cf323a98bfaa" />