> The killer app I'm trying to apply this to is LaTeX, so that I can write math notes in Emacs, incrementally, without visible latency.
See texpresso [1] for one solution that does something like this with the LaTeX process.
Another, more conservative solution is the upcoming changes to Org mode's LaTeX previews [2] which can preview live as you type, with no Emacs input lag (Demos [3,4]).
[1] https://github.com/let-def/texpresso
[2] https://abode.karthinks.com/org-latex-preview/