There is no option to center icons on tabbed docks at this time. Maybe in a future release (I hope).
I was just experimenting with the separators for tabbed docks. With version 1.5, separators (which are a kind of blank icon) can be moved in the same manner as the normal icons, until you move it to the far right. At that point you have to hold the 'Shift' key down to move it again.
