How about running it as a background to the website, over say, a month, at the end of which you'd have pretty good confidence that you'd have hit all paths.
This is an idea I am currently working on, similar to SitePoint's old "Dust Me Selectors" plugin for Firefox, but running in the background for all selected sites until a set amount of time or the user is happy that they have exhausted all dynamic functionality on the site.
How about running it as a plugin or something for a suite of functional specs for the website ( if it has one! ). Functional spec's by definition should run all the paths that a user can take, so it should give a pretty good idea of the css that is used.
Yeah, you could have a proxy that saves the whole HTML with its URL for every hit.
Then run an analysis after a couple of days and you should get a pretty good idea.
