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.
