The z axis is set by the modeller, its the projection plane poly and is positioned in the host CAD program, there is no other way to move it I'm afraid. The XML gauges are projected onto this transparent plane by the sim, it is very hard to ge the poly close to the panel to avoid parallax visual errors and to avoid it disappearing by z clipping from the sim. It is this plane that turns opaque on many prot overs and blots out fwd vision from the VC, this is due to the way FSx renders this screen differently to FS9, ie materials and textures....its actually an invisible texture applied to the plane that will be transparent...except for where the XML gauges are.
This is one reason why 3D gauges are popular, it avoids this small niggle in the VC, the down side of 3D gauges is that end users cannot swap gauges in and out.
Hope that makes some sort of sense ?. Its probably far more technical than that, but thats my ham fisted way of understanding it LOL.