As far as we see, there is a limitation that allows nothing else but adding a drop-down menu in some limited locations.
We need more flexibility.
(Drop-down menus are overkill as we want to add only a single entry point to our app, and adding a drop-down menu with a single menu item looks really lame.)
These are the most typical location where we want to add buttons:
the whole backlog (for Scrum and Kanplan, too)
each sprint separately in the backlog view (Scrum only)
the whole board (for Scrum and Kanban, too)
each column separately in the board view (for Scrum and Kanban, too)