With my ext app I can do your request but what appened if you add an other icon?
It's the same roblem of triangular shape, it lose the shape's coherency. The problem increases with more icon added and when there are 14 the center is lose in one of four center icons. All the non linear shape have this problem and are valid only for complete "octet" and in your case 8 icons, 24 icon ... (n^2 -1) where n is only odd.
I think for this particular shape is better to wait the docklets and if they are simple I'll made surely your square shape.