I found a nice Drafts 4 action here for previewing LaTeX as HTML via MathJax.
How would I go about modifying this action so that the resulting HTML preview can be imported into, say, Evernote?
PS: the Mac beta finally convinced me to get that year subscription 
I don’t think you’ll have much luck combining MathJax with Evernote. It is a dynamic javascript tool that has to run in the browser and does not just general HTML output that would be compatible with Evernote’s limited ENML markup.
All I could come up with was to render it to a PDF and then share that to Evernote. The rendering via Shortcuts though appeared to be hit and miss in my tests. But if you are okay with a PDF snapshot then that might be workable.
Any other method where you might embed I think you would have to convert the page to an image, crop it and then insert it into your note. I suspect this might be rather manual to accommodate any MathJax content.