Hmm, I see. A long long time ago, I used code from knitr to create a simple Shiny app, that is basically a live markdown editor (see: https://github.com/bborgesr/shiny_apps). You can try it out with shiny::runGitHub("shiny_apps", "bborgesr", subdir = "rmd"): the first thing it does is ask you to open a (.md or .Rmd file), then it displays a live markdown editor using that file as a starting point. As is, it's not saving anything, so it's not practical to use as an actual editor where you could save stuff (though that functionality shouldn't be difficult to add, though it's not a one-liner either). I suppose you can fork and build on that if you'd like, but it's a little old!
Good luck with whatever you end up choosing, sounds like a cool project!