Makes sense. Knowing MathJax, I was actually wondering how you managed to make the rendering so fast!
The PR looks merged, so \def and such should work. I am not sure how or where it would need to be injected for KaTeX, though.
EDIT: Not that PR, that one is not merged. This PR: https://github.com/Khan/KaTeX/pull/493 -- looks like macros of a sort can go into the settings?
BTW, I just want to say that you're pretty cool -- your responses are really useful and interesting! That is a welcome sight Keep up the awesomeness!