After getting pulled in different directions for a while, I was able to come back and work on TinyMCE. I upgraded the integration to pull the permissions for the text element via an AJAX call which was one of the last major hurdles to supporting in-line editing (see https://blogs.missouristate.edu/web/2008/11/03/tinymce-part-deux/).

The other hurdle was what to do when the element being edited was on the far right side of the page. The toolbar hanging off the right side wasn’t exactly the user experience I had in mind. I found some additional TinyMCE configuration options which let me size and position the toolbar independently of the editor area. So the final solution ended up using JavaScript to detect if the toolbar would fit, and if not have it shift left on the page.