Eric,
Just retried and was able to get it to work. Perhaps someone should add this to the ob-doc-latex page? For reference, I tried each of the following:
Works: 1) #+begin_src latex :headers \renewcommand{\rmdefault}{cmss}
Doesn't work: 2) #+begin_src latex :headers '("\\renewcommand{\rmdefault}{cmss}")
Works: 3) #+begin_src latex :headers '("\\usepackage{mathpazo}")
Which was puzzling me about #2 which led me to try:
Works: 4) Doesn't work: 2) #+begin_src latex :headers '("\\renewcommand{\\rmdefault}{cmss}")
Note the second double backslash before rmdefault. That escape character was necessary there, too.
Thanks for the help on this!! It's perfect.
John