Your idea sounds cool though possibly hard to clean. I am wondering at what point you would stop grouting. Maybe the wall and grout should be the same color to hide the transition. And it shouldn't be a straight line. And you could never repaint. I'm lucky if repaint every ten years lol so maybe that doesn't matter.
If you do it be sure to post a picture!
Kathy

I love the vision and think it can be done. I would keep the mosaic together, just diminish the number of pieces in a random pattern, and still grout it all together as one piece. If that makes sense.

why not? what you are imagining would fall under the category of strata in mosaics, look up the recent works of Kelly Knickerbocker at Rivenworks

Here is a link that might be useful: Kelly's strata work