Create a bookmark with the following URL:
(You might want to add it to your bookmark toolbar or assign a keyword for easier access.)
Now open an article, click "show N more comments", and then activate the above bookmark. MathJax re-processes all math in the comments.
Edit: Fixed; now it should work by design, not by accident. I hope. I'll CW this so that people who actually understand these things can provide a real solution.