Actually I've already suggested a mechanic that does what you ask for- "change speech bubble behavior so it disappears from map after triggered". This would allow you to put a trigger(speech bubble) blocking the way to the boss, trigger it by walking into it, get your boss taunt text and then the speech bubble would disappear, letting you pass and start the boss battle. It would also be great for environmental storytelling, without crowding the map.
Hope it would get implemented at some point :)