The struggle was long and bloody but I managed to find where and how a toolbar can be added. I had to dig through the cob_dashboard then through the rqt_robot_dashboard followed by a long stare at the Plugin class and finally ended up with PluginContext (see here)
Widgets are added through the context variable of the dashboard (context being of type PluginContext) using add_widget(QWidget) and specifically for the toolbar - add_toolbar(QToolBar).