The Heroku toolbar floats on top of your app, providing access to some development functions.
There isn’t a best practices sort of way to do this that I can see. But we did come up with a workable solution. Here’s a static HTML proof of concept:
There are some problems with this, such as the possibility of differing versions of Prototype in what the user’s app has included and what the toolbar code expects. My biggest complaint with it is that it’s just not that elegant. If anyone has other suggestions I’d be interested to hear them.