Um I would LOVE to see this feature. Currently, I have been naming the tags I use to classify notes within a single notebook/stack with this format:
[notebook/stack name]-[tag]
However, I find this tends to make my tag list look rather messy, and makes scrolling through the list of tags I can choose from on the mobile client much more of a pain than it needs to be. I've tried using more notebooks to separate my content; however, I again ran into the issue of a rather lengthy list of choices to scroll through when choosing where to save a note, particularly on a mobile device.
I've also tried using nesting tags, but found these were only useful if I was using the desktop client, which I rarely do. Having the ability to designate tags to be notebook and/or stack specific would be sublime... particularly if the mobile web clipper would then hide those tags unless I had selected a new note's destination to be in that notebook/stack. Also, it would be helpful to have those tags listed at the top of the note list when viewing only notes from that notebook/stack, for filtering purposes.