Green screen window non-square masking

[storrm]

Is it possible to add a mask to a chroma key overlay in order to crop around the main point of focus? I use a round green screen attached to the back of my chair therefore I need to crop the image in a non-uniform way i.e. cut out the corners. What I'm looking for is something similar to After Effects chroma key setup where you can mask the main subject.

Thanks