[Locked] How do I shade or paint "inside the lines" with Illustrator?

I'm a longtime Illustrator user, and have yet to figure out an elegant solution for perhaps my most common illustrator task. So, at long last, I'm resorting to the forums.

When one paints in Photoshop, it's common to use the magic wand (or some selection tool) to select a region so that you cannot paint outside of it. Let's say I want to add a shadow or highlight to a sphere -- I select the fill color of the sphere, and then can drag my brush along the perimiter of the selection, which keeps my shadow entirely inside the sphere. This is especially important for cartoon-style drawings where you want sharp shadows or highlights.

It doesn't seem there's any way to do this in Illustrator (using a freehand style of drawing, with a Wacom tablet or the like). The best solution I've found is to use live paint, which allows you to easily select any color areas that spill outside of a line (because live paint identifies those areas as separate entities). You can then simply delete the unwanted color. It's also possible to use the pathfinder tools to unify only overlapping areas, but this can get even more cumbersome if your illustration is complex.

My hope is that someone has a better way -- a plug-in perhaps, or something really obvious that I'm overlooking. Thanks you in advance. Also, I'm not interested in ways of doing this using the pen tool -- I know how to add shadows in that fashion. The goal is to find a way to quickly paint freehand, using a tablet and the brush tools.

P.S. I realize it's possible to simply bring the file into Photoshop, but then you're not creating a vector image, obviously.