I have managed to hack out a very crude plugin for the
markItUp editor. This is much like any RTE, only without the WYSIWYG part. No messing with MODx tags, no "helpful" cleanup of the HTML you want. A cross between TinyMCE and CodeMirror. Highlight text and click toolbar buttons to add HTML tags. Has image and URL insertion, too.
While I hope to refine it and get it working better, it does work, and just needs tweaking that I can't do right now (dratted eyes!). To-do includes adapting the includeJs() and includeCss() functions from the managermanager utilities functions. Also need be able to adjust the sets and skins via plugin configuration instead of hard-coding the paths. And of course fix it so that it's a regular recognized RTE for the site configuration (or user settings or whatever). And something's not quite right with the styling, but my eyes are on the blur again and it seemed good to throw it out at this point for others to pick up and run with.
Create a folder in your assets/plugins for markitup - assets/plugins/markitup.
In that folder, put the stuff from markitup - the .js file and the sets, skins and templates folders. Make your custom sets as you like. Make sure the main jquery library is available.
Paste the following into a new plugin, and check the onDocFormRender and onDocFormPreRender events. Edit the paths to the libraries, sets and skins (at least until the plugin configuration can be set up to do that).
Disable or turn off the regular rich text editor.
Anyway, this is more a proof-of-concept, to get it started, and certainly is not intended to be a finished product.