If you want to get away with stuff on a new build, its not hard. Yes, you have all your MEPs underslab inspection, and pre-pour inspection. Nothing stops you from changing things around post inspection, pre pour. No one will ever be able to see as all your piping is under the slab, and never visible again. You could then hide and cap the gray water, as you will need to fill the DWV for the rough in inspection. Its just more trouble than doing it per code
You would pretty much have to be an owner builder, though. Plumber isnt going to do it, GC isnt going to let the owner go dig and cut and move stuff on his house, etc.