thanks for the help! I found the following solution:
\override DynamicLineSpanner #'layer = #0
deh2~\<
\override TextScript #'layer = #1
deh16_\markup { \postscript #"2.5 -4 translate 6.2 4 scale 1 setgray 0 0 moveto
0 1 lineto 1 1 lineto 1 0 lineto 0 0 lineto fill" }
r8. deh8. deh16\sff bes4~
If someone doesn't know postscript: In order to find the right place
where to white out the hairpin I used a smaller number just before the
"setgray" in the postscript (0 means black, 1 means white). The two
numbers before "scale" change the x and y size of the whiteout
rectangle and the two numbers before the "translate" shift it into
position. If the whiteout has to be in the staff, I guess the
'extra-offset property has to be used.