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I don't know if this could be a valid answer for you, but how about this?

Instead of modifying openni_tracker you could write a node that listens to the user pose published by openni_tracker. The user pose is published as a set of transforms, one for each part of the body, (details here) in 3D space (in meters), so instead of a square you should check that all the parts of the body are inside of a cube defined by you and print your messages accordingly.

Hope this could help you.