I would suggest that you keep all the out-of-cakephp-framework related files, stored under the webroot.
My suggestion is the following directory structure for saving the html template as well as the css and js related files.
/webroot/files/templates/<template-project-name>/<the-actual-file>.html
Any external css, would then be under
/webroot/css/templates/<template-project-name>/<the-actual-css>.css
And the JS libraries would be under
/webroot/js/templates/<template-project-name>/<the-actual-js>.js
Use a seperate layout file (call it preview_layout.ctp
) which simply renders
the template file as it is.
Hope it helps