Even using SetObjectGlobalDepth() you will only get 3 layers (2Dback,3D,2DFront)
Paul has mentioned that he will reinstate the ClearDepthBuffer() for the next release of studio which will make this sort of rendering process easier. The command is still in AppGamekit classic and Is useful for mixing multiple layers of 2d and 3d.
I doubt that setting individual 2D depths for each 3D object will be implemented as it would not fit into the depth ordering in 2D at all, and would make the rendering loop pretty horrific to manage. Sprites and 3d are currently completely separate rendering processes and just will not mix at present without some considerable additional work.
There is another option though that could be used in the meantime.... You can set object depth ranges (setObjectDepthRange()) and make multiple render() calls to layer up so that you have 6 layers. It will involve moving cameras and changing view offsets between the render calls but it would work.