You're pretty close
When you put the CSS files in the right place, the magic happens when you send books to the device. Every time you send books to the device, before books are processed the extended driver will copy the appropriate CSS file to the device. Then, during processing, davidfor's code in the mainline driver will pick up the file and process it as if you'd put it there yourself.
The only thing to keep in mind, the rules are always added at the end of every stylesheet in the book. There's
a bug open right now asking to have an option to add rules to the beginning of a stylesheet instead.