Can I do some custom drawing on my control designer’s surface during design-time?

Yes, you can. You have to override OnPaintAdornments in your Control Designer. This will be called after the Control has painted.

A good example is when you have a Control that has its border style set to None, and you want to draw a dummy border in the designer.

protected override /*ParentControlDesigner*/ void OnPaintAdornments(PaintEventArgs pe)
{
	System.Windows.Forms.Panel panel;
	panel = (System.Windows.Forms.Panel)this.Component;
	if (panel.BorderStyle == BorderStyle.None)
		this.DrawDesignTimeBorder(pe.Graphics, panel);
			
	base.OnPaintAdornments(pe);
}

public void DrawDesignTimeBorder(Graphics g, Control control)
{
	System.Drawing.Rectangle clientRectangle;
	System.Drawing.Color bgColor, adjustedBgColor;
	System.Drawing.Pen pen;

	clientRectangle = control.ClientRectangle;
	bgColor = control.BackColor;
	if (((double) bgColor.GetBrightness()) >= 0.5)
		adjustedBgColor = ControlPaint.Dark(control.BackColor);
	else
		adjustedBgColor = ControlPaint.Light(control.BackColor);
			
	pen = new Pen(adjustedBgColor);
	pen.DashStyle = DashStyle.Dash;

	clientRectangle.Width = (clientRectangle.Width - 1);
	clientRectangle.Height = (clientRectangle.Height - 1);

	g.DrawRectangle(pen,clientRectangle);
	pen.Dispose();
}