From my notebook of experiments:
Please excuse the grim image.
A neat little trick when wishing to add more controllable areas of 3d to your 2.5d paintings is to implement the 3D plane tool rather than using some of the 2.5d tools.
Adding areas of z depth as I worked was done using the plane 3d tool set to a higher v and h divide level (initialize within the tool settings), > mesh edited to the form I required and then moved back into the canvas’s zdepth to only reveal the reliefed surface - this is then moved into position, sized etc and switched to zadd only before finally applying to the canvas.
The painting underneath will act as your guide to how you edit the mesh of the 3D plane tool.
Hope this is of interest to those of you new to ZBrush .