Similes of Attainment
There are two that come to mind from the Shoyoroku.  The first is Case 98 and it's entitled Tonzan's Heed.  In that instance, it more refers to reticence on the part of the teacher in not wanting to lead the student into intellectualism.  I'm not sure if that is the image you were after. The second is at Case 41:


  When he was about to die, Rakuho addressed his assembly and said, “I
  have one matter to ask you about. If you say ‘yes’ to this, you are
  putting another head on your own. If you say ‘no,’ you are looking for
  life by cutting off your head.”


I don't think this is what you are looking for exactly, either.  

I can't think of a single koan in the Mumonkan or Hekiganroku that has an image like that, however.  But kicked into mud?  Cat cut in two?  Finger cut off?  Nose pinched?  Now that's a different story!  

(Note: the Shaseki-shu has something about heads being cut off.  Unfortunately, that collection does not contain any actual koans.  Those are parables.)