i think you probably could if you made an image with some transparency to one side. for example you make a transparent square, place your image on the top left corner, save it as png and add it to GS. The image's center should be at the center of the "transparent square" you made... It's quite hard to explain!
You could make a small, invisible actor and treat it like your main one with all the rules and collision and all that. Then take a second actor with an image on it and constrain it to your invisible with an offset, like -50 in Y or something. That would effectively change the center of your object and it's fast to set up, but I guess it depends on what specifically you want to do with it. It worked for me when I was prototyping a 3rd person Monkey Island style perspective game and only wanted the players feet to collide with the world.
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It's quite hard to explain!