Looks like this will solve my problem. WP requires the domain live before the site is ready, so a Coming Soon page is perfect.
The site warned me it had not been tested for the latest WP and indeed I was unable to add images, except inside the page editing box. You could upload, browse and select but not choose it.
But for me, that's not a big deal. I just need the page with a bit of information and a link to the currently active site.