To answer my own question. The approach that worked was to just remove the "zNear" plane from the Polytope. It is the last plane.
Seems like an unnecessary hack.
PolytopeIntersector* picker
= new PolytopeIntersector ( osgUtil::Intersector::PROJECTION, nx1, ny1, nx2, ny2 );
auto planeList = picker->getPolytope().getPlaneList();
planeList.pop_back();
picker->getPolytope().set ( planeList );