I think it's a good idea. It could behave like an automation track. That way you could use automation points and curves instead of the rather cumbersome drawing tools that exist currently; it would make copying, pasting and moving tempo events and information around easier as well. You could use a fader to ride tempo in real time, if that made sense for some reason.
You could also have its display instantly toggle-able with a keyswitch, like the video track.