For the record, I'm not sure I approve. Writing the code horizontally does not seem to me much better than writing it vertically, and - in a friendly way - I am a little skeptical about this, in practice it may seem in practice. One of the joys of Python is that you no longer have to worry about writing GolfScript.
However, how about:
pyp "mm | p if n==0 else (p[:-2] + [(int(x)%12) for x in p[-2:]]) | mm"
which produces:
localhost-2:coding $ cat exam.pyp a,b,c,d,e,f,g,h 11,22,33,44,55,66,77,88 12,23,34,45,56,67,78,89 13,24,35,46,57,68,79,80 localhost-2:coding $ cat exam.pyp | pyp "mm | p if n==0 else (p[:-2] + [(int(x)%12) for x in p[-2:]]) | mm" a,b,c,d,e,f,g,h 11,22,33,44,55,66,5,4 12,23,34,45,56,67,6,5 13,24,35,46,57,68,7,8
[Disclaimer: This is my first ever pyp program that I downloaded about ten minutes ago.]