It's ironic that your avatar is Homer, because I just said D'oh!!
I have been doing a lot of work between AE and Premiere lately, so for some reason I was looking for the Crop that is in Premiere. I do remember a buddy of mine had a 3rd party cropper in AE that worked the same way.
Of course, masking is the obvious way! However, I am wondering why that effect is even called Crop Edges. But I'll research that on my own.