Yes, that was part of what I needed. I also needed to know what the default config was because I wanted to add a single button not start from scratch. I found this at
https://github.com/orbeon/orbeon-forms/blob/master/web-facades/src/main/scala/org/orbeon/facades/TinyMce.scala#L93 but it might be helpful to add this to the documentation. To make the change in Form Runner I needed to use TINYMCE_CUSTOM_CONFIG along with the oxf.fr.js.custom.uri property (
https://doc.orbeon.com/form-runner/component/rich-text-editor#configuration).
It has been a few weeks now so I can't remember, was there may have been a difference in behavior between the editor that appears directly on the form in Form Builder and the editor that appears on the Explanatory Text tab of the Control Settings dialog.
FWIW, as I was working on the form a college commented that it would be much more efficient if the control settings dialog was docked to the right side of the page. This had never occurred to me before, but since he said it I can't stop thinking about how nice it would be.