i think, it should be possible by adding some scripting to the footer. there is an example for syntax highlighting in editor fields in the documentation.

The new button could be added by a pre/post entry in the css for the editor box.  Eventually we have to add a new id to the color input. I´ll take a look at the sources.

Using css/javascript in the footer enables us to have a small binary and prevent any licence problems, because adding code is an user decission inside fossil configuration.