Sticking this after my initialization #+ stuff finally worked:

#+begin_html

<script type="text/x-mathjax-config">

 MathJax.Hub.Config({

   TeX: {extensions: ["cancel.js"]},

   tex2jax: {

     inlineMath: [ ['$','$'], ['\\(','\\)'] ],

     processEscapes: true

   }

 });

</script>

#+end_html

although I, the eternal beginner, don't quite understand other than MathJax is pulling in this cancel.js and applying it. Got it off the MathJax Google Group. I'm sure there's a better way to roll it into Emacs/org-mode or org-html-mathjax-template?