It's not a metaphor for anything. Carolyn Grantham reports on eating an actual pig's head at Estragon in the South End:
I like to be prepared for unusual situations, so I'd done some research into the best way to approach the task of eating this delicacy. But while I'd discovered plenty of advice on how to butcher and cook said object, there was precious little information on how to eat a pig's head.
And now here one was, eyeing me expectantly. ...
Complete with slice-by-slice photos.