This won't work without some code, and any CMS effort we work on should include at least some support for CSS files.
Dave, it's clear that leaving stuff in the back of your mind helps you understand the problem
I think what you are suggesting will work, but I'm not sure that the support has to be specific to css files. The css is really just a special case of the general problem of generating page contents based on location in a file system.
That being said, it _would_ be nice to have a UI that allows one to browse attribute/value pairs and assign them based on location in the published file tree.
ie, if you're asking for a css file for a file being published to foo, then make the P tags orange. If the file is published to bar, make them blue, and so on.