Following my first idea of a blah script for irssi, I've wrote the same thing in Python for filtering some mail. IMHO, it's better than killfile.

  1 #!/usr/bin/python 
  2  
  3 import email, sys, re 
  4  
  5 def blaahw(match): 
  6     w = match.group(0) 
  7     l = len(w) 
  8     if l == 1: 
  9         return "o" 
 10     elif l == 2: 
 11         return "he" 
 12     elif l == 3: 
 13         return "lol" 
 14     a = "a" * (l - 3) 
 15     return re.sub("\w+", "bl%sh" % a, w) 
 16  
 17 def blaah(text): 
 18     rc = re.compile('\w+') 
 19     return rc.sub(blaahw, text) 
 20  
 21 mail = email.message_from_file(sys.stdin) 
 22  
 23 mail.replace_header("Subject", blaah(mail["Subject"])) 
 24  
 25 def blaah_payload(pl): 
 26     if type(pl) == list: 
 27         for i in range(len(pl)): 
 28             if pl[i].get_content_type() == "text/plain": 
 29                 pl[i].set_payload(blaah(pl[i].get_payload())) 
 30         return pl 
 31     elif type(pl) == str: 
 32         return blaah(pl) 
 33  
 34 mail.set_payload(blaah_payload(mail.get_payload())) 
 35 print(mail)

Then can use a procmail rule like:

:0 fw
* ^From: asshole <asshole@asshole.com>
| $HOME/bin/blahmail.py

EDIT: I've updated it to a more robust version which support multipart messages.