Interesting question. I found this post and tested, it worked for me:
http://wpdreamer.com/2012/03/how-to-define-reserve-slugs-for-wordpress-posts-and-pages/
If you don’t want to edit your functions.php file an inelegant solution you can try is to create empty placeholder pages with the permalinks you’d like to reserve. If there is a physical directory on the server with that same name that would take precedence over the rewritten WordPress URL. So for example, if you create a WordPress page with slug “demo” and a physical directory in your web public dir called, “demo” you could use that for your demo site files.
Personally I like to use subdomains for this purpose myself to keep any demo work segregated from production, but that’s just an opinion.