Select all objects infront of or behind and object?

I don't think that zorder is going to prove what you want for this… It only allows you to move forward/backward with in a layer or group container… The easy way to select objects within a set of bounds is to make a temp artboard then use that to select the art… You would need to work through the selection array and unselect whats above…? A recursive top to bottom loop could be used… Never done this mind just out my head stuff…

This a script from James E Talmage

/*
JET_MarqueeDeselectPartials.jsx
A Javascript for Illustrator CS2 by James E. Talmage
Purpose:
When making marquee selections in Illustrator, all objects crossed or touched by the selection marquee are selected.
Unlike most other drawing programs, Illustrator does not provide a Contact Sensitive toggle setting to cause marquee selection
to select only objects which are fully enclosed by the marquee.
This script deselects paths in the current selection which are only partially selected.
To Use:
Make a marquee or lasso selection which completely surrounds the paths desired for selection. Then run the script.
Additional paths which were only partially selected become deselected.
*/
var docRef=activeDocument;
for(i=docRef.selection.length-1;i>=0;i--){
    var objRef=docRef.selection;
    var unSelecteds=0;
    if(objRef.typename=="PathItem"){
        for(p=objRef.pathPoints.length-1;p>=0;p--){
            var pointRef=objRef.pathPoints
;
            if(pointRef.selected!=PathPointSelection.ANCHORPOINT){
                unSelecteds+=1;
            }
        }
    }
    if (unSelecteds>0){
        objRef.selected=false;
    }
}
alert("Partially selected paths have been deselected.");

It may serve your purpose.