Problems with orient along path in 3d composition

Hi all, 

I have a composition running geolayers 3 and want to do a simple 3d camera fly in and have some objects enter the screen along bezier paths. The problem I'm running into is with orient along paths, which is obviously orienting the objects along every axis, when I only want it to orient along the z axis. I could just adjust the orientation manually but it's not as smooth and somewhat impractical with the many dozens of objects that the scene will have. I was wondering if there is an expression that can be used to restrict the orientation to the z axis alone (all the other similar questions asked on here and elsewhere seem to refer to auto orient to camera.)

I have attached three videos to help illustrate the issue. The first is an overhead view of what is happening with auto orient turned on. The second is the camera movement with one auto oriented object entering along its path. The third is the same shot with auto orient turned off. 

Any help much appreciated.