Constrain painting to a color or shape area

Question

In short, I want to be able to select an area of a single fill or shape -- Photoshop Style -- and constrain painting (with the Blob brush most likely) to within that shape. I want to be able to do this without creating a clipping path. This technique is used by Photoshop artists all the time for quick, precise shading and highlighting, and would be immensely helpful for artists who work in Illustrator, many of whom make cartoons. This would be a time-saving and workflow feature more than anything.
This is a feature I've long sought out, and actually had a long discussion in the help forum to see if such a feature already existed (http://forums.adobe.com/message/2586776#2586776). It was agreed by several of the participants that it doesn't, but that it would be useful.
When I shade in photoshop (say, a add shadows to a cartoon), I will use some selection tool, usually the Magic Wand, to select a flat color area so that I cannot draw "outside the lines." This allows me to quickly shade an area with a perfect outline that doesn't bleed into any other areas. Not only is this faster than using vector tools, it creates a more painterly or human touch to the shading, which is sometimes desirable.
In Illustrator, you currently have to resort to one of three options (generally speaking), none of which are fast or elegant, and become especially cumbersome with complex illustrations. The first involves using clipping paths to create a stencil of sorts. The second involves using Live Paint -- you draw with the blob brush outside the lines, and then select and delete areas that fall outside an outline. The third is to use the pathfinder to remove areas that fall outside an outline.
All of these methods take extra steps, can be slow, and involve clean-up. They also involve making duplicates of layers, which can bog down a computer if the file is too large. Given that Illustrator ALREADY can compute where an area of a certain fill begins or ends, or where a shape begins or ends, it would be nice if it would automate this process of constraining painting to a given fill or shape area.

Answer 1

Good question Doug (I'll post the response here as this thread is more relavent at this point). I believe I understand the thrust of your question, but apologies if I'm missing the point.
There are actually a few reasons why I would prefer the solution I am requesting.The main reason is personal, and it's one of workflow. I'm simply more comfortable and faster when drawing brushstrokes with my tablet than I am at manipulating pictures using vector tools or inputting settings (say manipulating a gradient, or pre-visualizing an appropriate tinting and then applying it). I like the steps Adobe has taken to push Illustrator in a more organic direction, and this would further that. And this isn't a purely superficial issue -- as an artist, your thought process is intimately connected with how you work, and not being able to work in a way that's comfortable to me impacts my ability to reach a satisfying end product. Using a marquee in Photoshop is less of an interruption to my brushwork than is manipulating a style in Illustrator.
From a purely technical perspective, it is actually kind of hard to achieve good cel shading using a shaded fill, though I'm actually glad you brought it up as it hasn't been mentioned once in the discussion thus far. I'm talking about creating the type of illustration you might see on a graphic t-shirt, where all fo the shading is done in a solid color. I don't actually mind that the shadow or highlight is a separate object -- this is actually a good thing. It's just very helpful to be able to swoosh my brush over the edge of an object (as one might an airbrush) and have a pretty looking shadow right where I want it -- AND that I can then tweak as it's a vector object.
What I do mind is the amount of labor it takes to get the edge of the shadow to adhere to the outline of the object I'm shading. No matter how you slice it, there's at least one more step than there should be, usually two or three more.
In short, it's really neat that Adobe has made vector art creation, a traditionally technical process, a more organic one like painting -- adding what amounts to a quick stencil feature (how I'm using marquees in Photoshop) would take this a step further for some artists (like myself).
I hope that answers your question, though possibly not.