I think you guys are going down the wrong track.

On Tuesday, November 8, 2011, Marius Hofert <marius.hofert@math.ethz.ch>
wrote:
> I just figured out that if I start emacs from the Mac's terminal (via
"emacs" which starts /Applications/Emacs.app/Contents/MacOS/Emacs) and open
an emacs shell with M-x shell, I can tab-complete a2ps.
>
> Hmmm... so the GUI version seems to act weird...

It sounds to me like OSX's launcher is not setting some environment
variable that does get set when you run the terminal.  Possibly $FPATH.  I
would guess that it's looking for and not finding a function called _a2ps
to do the completion.

-- 

-PJ