That's an interesting option, however integrating it with the file manager may prove a little trick. If you look at a simple editor like ACE (
http://modx.com/extras/package/ace) it should get you started down the right trail.
In regards to integrating with the file manager I would suggest checking out TinyMCE (
http://modx.com/extras/package/tinymce), which on a side note can be limited to the buttons you want for the user.
If you just want to change the default buttons in your System Settings choose "tinymce" under the core dropdown. Scroll down to the Custom Buttons Area and change the the five rows as desired. The second example (demo 1) could be accomplished by putting the following for the first row (others blank)
bold,italic,underline,justifyleft,justifycenter,justifyright,justifyfull,bullist,numlist,formatselect,outdent,indent,image,modxlink,styleprops
You don't get the font size or font family dropdowns, also the two color pickers aren't offered directly, however the last item on the list gives you control over those items and a little more.
I hope that helps some