A kind soul on the Cocoon list pointed out a missing variable in my XHTML code (the "realpath" component was missing from the path to the folder I was trying to create -- a dumb typo), and I've now got XHTML backups working fine, including saving copies of the CSS and JS into the backup folders, to create independent packages for each document.

The next stage is to make this all function through AJAX. This isn't too complicated, but it will require some thought; I need to return some useful content (the date/time of the file created), in a usable format, to replace the current info on the page, so I'll need to read that using Java once the file is created.