Вот решение @ bstoney, реализованное в коде.
Pen path_pen = new Pen(new SolidColorBrush(Colors.Red), 0.2);
path_pen.EndLineCap = PenLineCap.Square;
path_pen.StartLineCap = PenLineCap.Square;
Point path_start = new Point(0, 1);
BezierSegment path_segment = new BezierSegment(new Point(1, 0), new Point(2, 2), new Point(3, 1), true);
PathFigure path_figure = new PathFigure(path_start, new PathSegment[] { path_segment }, false);
PathGeometry path_geometry = new PathGeometry(new PathFigure[] { path_figure });
DrawingBrush squiggly_brush = new DrawingBrush();
squiggly_brush.Viewport = new Rect(0, 0, 6, 4);
squiggly_brush.ViewportUnits = BrushMappingMode.Absolute;
squiggly_brush.TileMode = TileMode.Tile;
squiggly_brush.Drawing = new GeometryDrawing(null, path_pen, path_geometry);
TextDecoration squiggly = new TextDecoration();
squiggly.Pen = new Pen(squiggly_brush, 6);
text_box.TextDecorations.Add(squiggly);