First, great idea. I've thought especially when placing a mainbus how useful something like this would be.
Although it requires research, I discovered its possible to "Brush" belts..or anything for that matter in vanilla, with blueprints click and drag. So one could just create a x width Belt Blueprint and then brush/paint it on as desired. IMO Placement via ghosts (Shift+Click) is preferable since you can stay in one place and just paint anywhere construction robots can access.

yes, this is very true, and in late game also quite useless as you have bots. But sometimes bots are slow, especially when just making your bus.