Integrating mathjax into x: head.js - mathjax/MathJax-docs GitHub Wiki
It looks like
head.js("http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML");
works (I tried it out). You could also load your own local configuration as
head.js("http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML,http://mysite.com/mathjax/local.js");
in order to not have to have in-line configuration script blocks.