Tengo este método en mi clase de texto y parece que no puedo voltear todo el texto.
Estoy usando una matriz para transformar laGraphicsPath que se usa para dibujar la cadena.

Aquí está el código desde que usé la respuesta de @ Jimi:

    public LayerClass DrawString(LayerClass.Type _text, string text, RectangleF rect, Font _fontStyle, Brush brush, float angle, PaintEventArgs e)
    {
        using (StringFormat string_format = new StringFormat())
        {
            SizeF stringSize = e.Graphics.MeasureString(text, _fontStyle);
            rect.Location = new PointF(Shape.center.X - (rect.Width / 2), Shape.center.Y - (rect.Height / 2));
            GraphicsContainer gc = e.Graphics.BeginContainer();
            e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
            e.Graphics.CompositingQuality = CompositingQuality.HighQuality;
            //e.Graphics.DrawRectangle(Pens.Red, Rectangle.Round(rect));

            RectangleF r = new RectangleF(rect.Location, rect.Size);
            GraphicsPath path = new GraphicsPath();
            if (text == "" || text == " ")
                path.Dispose(); //Disposes the path to prevent OutOfMemoryExcetption
            else
            {
                path.AddString(text, _fontStyle.FontFamily, Convert.ToInt32(_fontStyle.Style), _fontStyle.Height, new Point(0,0), string_format);
                RectangleF text_rectf = path.GetBounds();
                PointF[] target_pts = {
                            new PointF(r.Left, r.Top),
                            new PointF(r.Right, r.Top),
                            new PointF(r.Left, r.Bottom)};
                //e.Graphics.DrawRectangle(Pens.Red, Rectangle.Round(r));
                using (Matrix m = new Matrix(text_rectf, target_pts)) 
                using (Matrix rotate = new Matrix())
                using (Matrix FlipXMatrix = new Matrix(-1, 0, 0, 1, 0, 0)) 
                using (Matrix FlipYMatrix = new Matrix(1, 0, 0, -1, 0, 0))
                using (Matrix TransformMatrix = new Matrix())
                {
                    TransformMatrix.Multiply(m);
                    m.RotateAt(angle, new PointF(0 + (stringSize.Width / 2), +(r.Height * 2)));
                    rotate.RotateAt(angle, new PointF(r.X, r.Y));
                    TransformMatrix.Multiply(rotate);
                    if (flipped)
                    {
                        TransformMatrix.Multiply(FlipXMatrix);
                    }
                    path.Transform(TransformMatrix);

                    if (flipY)
                    {
                        TransformMatrix.Multiply(FlipYMatrix);
                        path.Transform(TransformMatrix);
                    }

                    //Checks if the user wants the text filled or outlined
                    if (!isOutlined)
                        e.Graphics.FillPath(Brushes.Red, path);
                    else
                        e.Graphics.DrawPath(Pens.Red, path);
                } 
            }
        e.Graphics.EndContainer(gc);
        }
        this._Text = text;
        this._TextRect = rect;
        this.brush = brush;
        this._Angle = angle;
        return new LayerClass(_text, this, text, rect);
    }

Ahora el problema es que sale del centro del cuadro de imagen.