Perhaps TeX to MathML javascript library might allow things to work with both a MathJax 3 and a Mathjax 2.7.x polyfill.
Maybe something like:
https://github.com/ronkok/Temml
UPDATE,
After looking at it a bit more it seems the Readium reader just like Sigil only injects a link to its internal MathJax when it detects a mathml math tag on first load of a page. Without a mathml math tag being present, MathJax is never loaded.