I decided to see how it would look if I cut out rectangles and used the cookie cutter as part of the decoration/frosting rather than the shape. Again, this was just a test so the rectangle isn't perfect.
In another test, I cut out some hearts and frosted them in red. After they dried, I took the key cookie cutter and just "cut" a key-shaped outline in the center of the heart. (I tried to get a picture but I couldn't get it to come out right so you can see the key in the middle of the cookie.) It's not bad, but I like the rectangle cookie above better. I even like the ragged edge of the key. I think it's fitting for Halloween season. :-)
I've got another recipe to try (thanks Heleen), so I'll keep you posted.