While I don't know of a way to do exactly what you're asking, a simple way to achieve the same result would be to make a new artboard for this one shape. There are two methods for this:
One is to select the artwork and go to Object->Artboards->Fit to Selected Art. This will resize the current artboard to the selected art.
If you don't want to change the current artboard, you can use the shortcut Shift+o to start editing the artboards, then single click on the shape you want. This will make a new artboard, sized to this shape. When you are done saving the art, you can hit Shift+o again, and remove the artboard you just created.