I found this: https://panel.dreamhost.com/kbase/index.cgi?area=142
Which says "Viewing your site before your domain is up!
If you'd like to view your uploaded site on our servers (to check that everything is working, or whatever!) before your domain is up, you can! Simply go to the Domain > Add Domain area of our web panel and add a free dreamhosters.com sub-domain (e.g. "yourdomain.dreamhosters.com"). Choose it to be a "mirror" domain and set it up to mirror your real domain that hasn't gone live yet. When you're done, within an hour you'll be able to see your site at http://yourdomain.dreamhosters.com/ exactly as it will be when your domain is transferred!"
I've signed up, we'll see if it works.