Actually I don't think they could copyright the steps of the "dance" itself, just the name, as far as I understand it. (I believe you can not copyright, movement and the human form in the US.)

Theoretically if you made the shirt, with a generic human form (think restroom signs, etc) and didn't specifically say it was the Manderville, or include their logos, you could actually sell it. However if you used the name and an image from the game/logos you'd run into trouble.