I’ve started a project/exercise here
Vau aims to factor the TeXmacs typesetter away from the UI. One possible use is just to have a small viewer which can produce, e.g. png images of pages, to be used for example as plugin in other softwares. The main goal is however to allow me to understand better the dependencies in the code and attempt to refactor them for better modularity. Also I imagine a browser app where the UI is written in Javascript (or Elm or whatever) which communicates with the editor on a separate thread.
In the project I also experiment in rewriting the glue code to Scheme via C++ templates and make it more modular.
This is an experiment and it is not yet working properly.