Corrections to my script:
var myBleedRectangleOutside = myDoc.rectangles.add();
myBleedRectangleOutside.geometricBounds = [0, 0, myPageHeight, myPageWidth];
myBleedRectangleOutside.fillColor = myDoc.colors.item("Green");
var myBleedRectangleInside = myDoc.rectangles.add();
myBleedRectangleInside.geometricBounds = ["0.0625 in", "0.0625 in", myPageHeight - 0.0625, myPageWidth - 0.0625];
myBleedRectangleInside.fillColor = myDoc.colors.item("White");
Actually I am even not sure if I need a fill color for my inside rectangle, because I want that part to be transparent when compound.
"subtractPath", as in
It's the reverse of what I would thought it'd be, but it seems to work; switching 'inside' and 'outside' gives an "empty path" error.
The bottom and right hand edges are mere lines, you need to make the bleed smaller by twice the necessary amount -- once for the left side, once for the right side.
That's perfect, thank you very much.