Right now, the docs (http://www.mediawiki.org/wiki/Extension:WikiEditor/Toolbar_customization#Modifying_things) say:
"We don't really have a nice API to modify things, unfortunately"
An API for this would make ProveIt (an extension i work on that does this) cleaner and more reliable. Right now, there seems to be a race condition where sometimes it works, and sometimes it doesn't.
The workaround in the docs does not make clear whether to wrap the modification bind in mw.loader.using( 'ext.wikiEditor.toolbar',. I think not, because then it might be too late when the modifier's (ProveIt's) code runs.