Code to place a swatch pattern on a layer

I recently created a script that reads the layer names of an Illustrator file, and uses the name to identify a swatch to use to pattern all elements on that specified layer. The script works well, and is a real time saver for our mapping staff, but it requires duplicate swatches to exist, where the only difference is their colour.

To improve the script I want to duplicate a swatch and modify its' colour before applying it to a specific layer. I think the only way this can be done is via the "placedItems" command. So the code I'm trying to write looks likke this:

for(var s=0; s<slib.length; s++){                                // search swatch library

    var pat=slib[s].name;                                             // get swatch name

    if(pat == patTile){                                                   // does it match specified pattern

        //match found...place on Temp_Pattern layer

        var placedTile=doc.placedItems.add();             // my attempt to place swatch on a specied layer/location

        placedTile.file=doc.slib[s];

        placedTile.layer=doc.layers[0];

        placedTile.left=0;

        placedTile.top=0;

    }

}

However I'm struggling to find the correct syntax to make this work. Can someone help me?