It would be nice to have the option to use bordermatrix in MathJaX. See the bottom of the page http://www.math.harvard.edu/texman/node25.html
There is this recent (June/13) news from mathjax.org: Stack Exchange continues as MathJax Partner posted by Peter Krautzberger.
They appreciate Cross Validated SE feedback and the following news excerpts reinforce the advices given by @cardinal and @Abby to ask for support (feature-request) directly to MathJax team:
Stack Exchange continues its support as a MathJax Partner, demonstrating its commitment to being a partner to the math and science community on the web.
MathJax continues to benefit from Stack Exchange’s help with pre-release testing and we’re grateful to get the feedback from their technical team as well as the community which keeps MathJax flexible, open and robust.
We look forward to continuing the collaboration with Stack Exchange, and welcome their ongoing support for the MathJax project.