In order to make parts of a crescendo hairpin invisible, the following method is used: A white rectangle is drawn on top of the respective part of the crescendo hairpin, making it invisible. The rectangle is defined as postscript code within a text markup.
The markup command
with-dimensions tells LilyPond to consider only
the bottom edge of the rectangle when spacing it against the hairpin.
staff-padding prevents the rectangle from fitting
between the hairpin and staff.
Make sure the hairpin is in a lower layer than the text markup to draw the rectangle over the hairpin.