Putting a Site on Hiatus
 

For a special promotion, I want to hide all old posts for a time and have just one single home/landing page.

I get a lot of traffic from Google searches and a fair amount from links. I'd like those folks to get the landing page, not a 404 page. Is that legal on the internet?

Other than putting all posts and pages in draft mode, how can I accomplish shutting down the site?

I wouldn't recommend this, but you could do a redirect in your .htaccess file pointing all teh pages to teh front page. If you opt to do that you should use a 302 redirect.
http://www.internetofficer.com/seo/302-redirect/