If you have a report section that is large enough, you can
just use a large label control with an appropriate font size
and fore color of light gray.  Set all of the controls
BackStyle to Transparent and use Format - Send to Back to
place the big label behind all the other controls.  To show
or hide the label, just add a line of code to the section's
Format event:
    Me.thelabel.Visible = (Me.shipfield = "overnight")

An alternative is to use a large bitmap image as the
report's Picture property.

Create a graphic that looks like what you want. Place this in an image
control on the report. On the Format menu, use Send to Back to place this
behind the other controls on the report. In the Format event of the section
you can enable or disable this control based on the value of your field.

To make the graphic, try using Word Art in Word. You may also find this
example by Stephen Lebans helpful, http://www.lebans.com/xrotatetext.htm. If
you don't want it diagonally across the page, just a regular label will
work.