I would use robots.txt to noindex the whole website as well - but just the new pages, not the old ones. Then when you're ready to be crawled, remove the robots.txt entry and Fetch as Googlebot to get re-crawled.  You may fall out of the index for a day or two but should quickly be re-indexed.

Another solution would be to use the meta robots tag to individually noindex each page (if there's a way to do that in your CMS, obviously adding them by hand wouldn't be scalable), and then remove.  That may increase your chances of getting re-crawled and re-indexed sooner.