For completeness, here is my final code based on Charles's suggestion:

#+begin_src org
  ,* date formatting with version information               :ignoreheading:
  ,#+NAME: mydateline
  ,#+BEGIN_SRC emacs-lisp
  (format "#+DATE: \\copyright{} %s\n" *this*) 
  ,#+END_SRC

  src_shell[:post mydateline() :results raw]{echo -n $(date +%Y) '@@latex:\\ \footnotesize \color{gray}@@' v$(hg id --num)} 
#+end_src

This is for export to beamer (well, LaTeX more generally) and outputs
the year with revision information below in light grey (British spelling
I'm told ;-)).  I should probably move the copyright symbol into the
shell script but this works...

Thanks again,
eric

-- 
Eric S Fraga, ericsfraga@<most social media>