A good idea. I only got the group when I inserted the clip even though there was other code in it. Haven't had a close look why though.
I simplified this a bit to use a single BW image as a surface layer mask. This way you don't need to warry about transparency. The grids are semi-transparent by dropping the coverage below 0.5
You can add finer grids to heart's content for working at different detail levels by simply duplicating a surface layer/image map pair and changing the scale of the image map. The final node is not really required, I just added it as a connection point. Whose up for a functional version?