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Check AEScripts.com and AEnhancers.com. there's expressions and scripts for this sort of thing.
At AEScripts.com, I found a script called pt_Multiplane by Paul Tuersley. Although it wasn't 'draggable' like the Pan Behind tool, with a little bit of pre-planning and experimenting I was able to manipulate the layers I wanted quite easily into the positions I was after with this script. At this point I would be happy to say that Paul Tuersley has done a very nice job on this, it is certainly useful for me. Thanks to him for his work on this script and to you for pointing me to a great source.
I'd use a simple expression for the scale property. You could then adjust the position of the layer or the position of the camera and keep the layer the same size in the frame. Changing the anchor point in Z would then also change the relative size of the layer in the camera view. I've written out the expression in long form so that it's easier to understand. It uses the length (distance) between the 3D layer and the camera/divided by the zoom value (focal length) to create a scale multiplier. Here you go:
point1 = thisComp.layer("Camera 1").transform.position; point2 = position; d = length(point1, point2); z = thisComp.layer("Camera 1").cameraOption.zoom; sa = 100 * d/z; //scale adjustment [sa, sa, sa]