I haven't touched WP in a few years; does it have the equivalent of the Rails Asset Pipeline yet?
*This plugin hasn't been updated in over 2 years. It may no longer be maintained or supported and may have compatibility issues when used with more recent versions of WordPress.
As for the per-post CSS files it would be easy to do with a Posts custom field. You could take text input (CSS) and spit it out in the header using a basic query on the current post.