Hi Scott,
That's a great idea! We'll add it to our enhancement list.
In the meantime, you can add a shape to one of your Masters, and turn off the Start Visible property, so it will show up while authoring, but never at runtime. See attached image. It's the same size as my stage, it has a Fill of "none", and thick dark line, so I can see it on other pages. Doesn't really matter if it's above or below the page level content (since the Fill is set to None). You can also Lock it in the Cast, so you don't accidentally click on it.
Hope that helps!