I love this idea, and I'm wondering if it will work...
I've heard of putting a rope with bells on the door so that the dog bonks the rope, bells ring, and you know the dog needs to.. ahem... go outside. My friend does this with her dog with good results.
But my worry is that it will get annoying, having that bell ring every time we go in and out the door. I wanted something only Maddie would do, so she could really associate BELL with OUTSIDE (though she may associate it with WALK, and then we'd hear BELLS all the time).
Then I found this little bell. The idea is adorable.