Check out https://github.com/mathjax/asciimathml/blob/master/asciimath.... It uses the AsciiMath parser to generate latex to be fed into mimetex for rendering, but it could probably be daisy-chained with katex. I keep it current with the latest AsciiMath revisions.
I created such a daisy-chained page that uses ASCIIMathML input with KaTeX rendering where possible, and it falls back to MathJax when KaTeX can't handle the expression. See
