[BillC]

I got the Zoombox working for a plane within 3D.  In my case I'm cutting a cross-section of data then decorating it with a cross-section grid.  The cross-section lines are actually in 3D within the data space.  This allows the user to view it all in 3D and zoom into it if they wish in order to put it all in context with the bigger picture of the data.  However in the Cross-section view I only show the cross-section data and decoration.

I solved the problem by calculating the Zoombox center as if the viewbox was in the XY plane at the origin then applying a 3D rotation and translation to move the camera to the actual cross-section data.  In my case I do not allow the user to do a rotation within the Cross-section view.